Death can be an irreplaceable motivator. It can also be an enabler. It can also be the reason for a person's personality and outlook on life.
In "Sleepless in Seattle", the entire story hinges on the fact that the male lead's wife has died, and his son needs a new mother. She has to be dead, not just disabled, not just divorced, to enable the plot.
In one of my stories, the female hero has a first love and first lover that dies, when they are teens, in part because of an impulsive decision by her. She holds him as he dies, knowing her impulsive decision is the reason he is injured, and dies.
No, they can't just break up. Her motivation in the story depends heavily on her guilt; the entire course of her life is changed by her guilt, in a way that is implausible for any break up. We can get over break ups. We don't get over causing somebody's death, a sensitive person can carry that guilt to the grave.
It is possible you can think of a way around killing a character, but it may come across as unrealistic. Sleepless in Seattle is an example, the story would have to be completely different if the wife of Tom Hanks was alive and well. We just don't have the same sympathy for a divorced Dad as a young Widower, and why is he "sleepless" if not grieving? Why would an engaged Meg Ryan obsess over him?
Sleepless in Seattle is unrealistic if the wife of Tom Hanks is alive; it is a different story requiring much more explanation to be plausible.
And the same is true in many stories, death is the ultimate motivation, and for villains, the ultimate punishment, cheered if they are particularly vile and harm innocents, like women and children.
The psychology of an audience is not necessarily the same as their real-life philosophy. In fiction, it is okay to harm the bad guys, especially when the author goes out of their way to provide the proof that the bad guys are bad guys that harm innocents. So harm in return is justified.
In real life, we are seldom offered the same conclusive proof, and so our opinions are changed or modified. Just in case we are punishing an innocent person, perhaps we should not be so harsh, and should focus on prevention of further harm instead of focusing on vengeance.
In fiction, you can justify vengeance. And you aren't really hurting anybody if you kill a character. If you write well they may feel real, but they are just a figment of imagination. No actual persons or animals were harmed in the writing of this novel.
Feel free to devise characters to be killed that will arouse the emotions of the audience. Mentors, lovers, innocents, even children. Few things can be more life-changing than the death of a loved one, and what we write about is often the highs and lows of life. Often exaggerated.