In my story, a homicide takes place. I want to describe this homicide at the start of the story, through the eyes of the victim, to foreshadow what's about to come. But victim and suspect are both unknown and unrelated to my protagonist. Should I put this homicide in chapter 1? Or is it wise to start chapter 1 with my protagonist, and describe the homicide in a later chapter (chapter 5 or so)?
Either can work really well! The thing to pay attention to is that each option builds a different sequence and experience for the reader -- so you want to consider which of the two choices works better for your story.
A good general guideline is that at the beginning of the book, the reader is looking for "what is this book about." They'll latch on to, essentially, the first promise of trouble, conflict, or theme.
So, if you start off with a murder, the reader will probably understand the story as being about the murder. They'll expect the story's resolution to be "the killer has been found and brought to justice," or something similar.
Whereas if you start off with a character, the reader will probably understand the story as being about the character. "Does the character find the bad guy" is one resolution, but so is "Does the character compromise his beliefs," "Does he protect his loved ones," "Can he outsmart his nemesis," etc., etc. In a murder mystery, any major question will probably still be resolved by solving the murder -- but these issues, for the detective, give the mystery's solution significance. The mystery feels more personal to the reader, because it affects the protagonist they care about.
A thought that might help: In a thriller or adventure story, a lot of the tension is often how important the threat is; how dangerous; how thrilling. In that kind of a story, this kind of "prologue" can establish what dangerous territory the story will be sailing towards, and can help set up threatening or intriguing elements that the protagonist themselves won't reach for some time. This kind of opening establishes danger; sets up the situation.
Whereas in a story that's more mystery- or character-focused, you probably want the weight on us connecting with the detective. Setting up who the character is, why this mystery carries personal significance, helping us see the world through the character's particular point-of-view, and what the stakes are in their eyes. This kind of opening establishes character, and often theme.
Obviously, most murder-based stories will have both -- characters and suspense! But, usually, one will be "more important," the thing the story is "about", and the other will feel more secondary, subordinate. So the important thing is understanding what kind of story you're telling, and then you can look at how to create that particular focus.
Those are your basic considerations; that's how you can make a decision that's right for the particular story you're writing.
Hope this helps :D
I would not put the homicide in Chapter 1.
I believe you are making the mistake of many beginning writers, thinking that you have to get to the action and the main conflict quickly to hook the reader.
That is not true. The vast majority of successful stories use the opening scenes, even 10-15% of the entire story, to introduce the main character, operating in their normal world, and NOT dealing with the "big problem" that the book is about.
In most successful stories, we first learn about the main character(s) in the beginning getting through their normal life, usually dealing with problems or irritations but not the life-changing problems the whole story is about.
Remember Die Hard? All the things we remember are big action scenes -- but the movie opens mildly; Bruce Willis is separated from his wife and kids, and clearly struggling with that (not too competently) for several minutes before we even get a hint he's going to have to go through a gauntlet to save them and the world.
Same thing with The Bourne Identity; but they have to give the MC amnesia so he can have a "normal" life of being himself before he turns into a superhero.
Spiderman doesn't start with Peter Parker being Spiderman, it starts with him being an average awkward kid, pining after a girl out of his league.
The Lord of the Rings opens telling us about Hobbit life.
You have to build sympathy and knowledge of your characters.
You want readers to care about what happens to them. If you jump into a big drama scene (like a murder) they don't care, they don't know who is getting murdered, if they deserve it, or who they are rooting for. Now of course, if you have a child or defenseless young woman getting murdered, they know, but they don't know enough to start with this, all you will generate is the general (and forgettable) sympathy we get when watching the news and seeing strangers that have been victimized.
When you get to the big drama scenes, you need your readers to know the characters so they know the stakes of what has happened.
That is what the first 10%-15% of the story is there to do:
1) introduce the main character, in their normal world, preferably doing something to deal with a normal world problem.
2) introduce the "normal world", anything special about it (if there is magic, or the level of technology, from near-zero to super-futuristic, etc.)
3) Along with (2), introduce the specific setting for the story; the hero lives and works in NYC or London, in the year 2083.
After that opening, there is an "inciting incident", whatever eventually launches the MC on a new path in life. By the end of ACT I, (it is about 25% of the story), the inciting incident has grown enough to push the MC out of their normal world to deal with the issue which is what the story is about. Note the inciting incident can be anything from an atomic bomb going off (in an action film) to first meeting a future spouse. In "Taken" (1st), we see a lot of normal life of a man and his daughter who is going on a trip to Paris or something, a lot of normal world before she is kidnapped into the underworld of sex trafficking.
Do not assume audience "knows" a father loves his daughter, or the daughter can trust her father to save her.
Do not try to get to the inciting incident (in your case probably this murder) too early. An attempt to hook the reader fast on drama will likely fall flat; you hook the reader fast by presenting a character with some minor problem that interests them; this gives the character something to be doing, and we like to watch people dealing with issues. All you need is this little thread to get them turning pages to find out what happens next, to get involved with your character. You don't need a big set scene like a murder or the aliens landing. The whole planet may be doomed, but we won't care if you don't show us somebody in it we want to live.
Remember, readers don't mind reading! They know something big is coming, but they will read along getting to know the character first. You have some leeway. They expect to read about the normal world first, that's how most stories work.
Starting with a story or event(s) that directly affect the protagonist would make the reader sympathize for the protagonist and every action they take.
If you mean to put in an element later on in the story that would be more bizarre, I would suggest you introduce the homicide in the later stages and not in the beginning. Again, if it were a movie, starting with the murder would surely be captivating but I wouldn't care much about the protagonist or in fact every other person involved in the crime scene. Rather, the action happening would definitely be something that would catch my attention. Getting an audience into the protagonist requires some context and a backstory. This is what I feel :)
You don't have to put the homicide into Chapter 1, but you might want to. While this approach is a well-worn cliche of police procedurals, especially series (in which the protagonist is known in advance), it is nevertheless an excellent way to lead into introducing the protagonist:
An injustice has been done. The world cries out for a hero.
Then you go on describing the hero's daily life, dropping breadcrumbs and red herrings to lead him/her to the main conflict.
Leading with intriguing events can help grab and maintain readers' attention.
Readers will be spotting the breadcrumbs and feeling clever.
Dramatic irony -- the hero makes an observation the reader already knows is wrong, misses one of the chances to get involved -- works on the first readthrough.
Readers are instantly invested in the hero because they want someone to set things right.
The initial sequence must matter in the larger scope, which can potentially make you show your hand too early.
Some readers may grow impatient with the pace of the story because they know where it's going and would rather it got there already.
Some readers may feel cheated that the first point-of-view character gets killed for "cheap thrills".
Victim and suspect are both unknown and unrelated to my protagonist.
It's because of this that the answer is maybe.
Sometimes a prologue (not Chapter 1) with unrelated characters helps set the tone of the book. The reader knows in the back of her/his head "this is a crime novel" (or mystery, or detective, whatever it is). Then, when the protagonist's life intersects with the murder, it won't come out of the blue.
An example of this type of foreshadowing comes from Game of Thrones, the first novel in the Song of Ice and Fire series. The prologue is about 3 men who encounter monsters in the forest. Two die horribly and one escapes.
Then the book turns to the everyday life of the Stark family. Chapter one includes the punishment of the aforementioned survivor for deserting his post (but as an uncommon event, not as unordinary). The characters don't know why he did it, but the reader does, though we don't understand it yet. But really, it's about normal life, the give and take of the characters. The 3 men from the prologue aren't important.
This sets the book up as fantasy with a horror element. We learn straight off that something terrible is lurking and dangerous, even many miles away. But we don't dwell on it. In fact, by the time it comes up again—much much later—we have already forgotten it. Learning about it anew reawakens our memory and the insider knowledge we have allows us to look at the characters a different way. We know it's real. We know it's important. They don't.
Prologues can serve another purpose. They allow the novel to break free of the usual point of view character(s) to show something that character/narrator could never know. Information that is hidden from the MC. In my own novel, I have a prologue about the childhood of the MC's grandmother. While the family knows the outline of what happened to her, she never talks about it. I use the prologue not just to tell her story but to inform the reader of the history all the characters deal with on some level.
I've seen novels like what you describe: They start with a murder or other crime/significant even then turn to unrelated characters. Later—sometimes much later—the crime becomes important.
This can work well if done carefully. If it's there just to be a hook, forget it. But if it's to help frame the story, then it can be useful.