My protagonist is a very shitty person that makes bad decisions for the wrong reasons. They do eventually redeem themselves, not because of their own decisions, but because the supporting characters push the protagonist in the right direction. Will a protagonist like that alienate the reader?
It is not and never has been about making the protagonist likable. It has always been about making them recognizable. If you want a great example of an unlikable protagonist, try Graham Greene's Brighton Rock. Pinky is in no way likable and in no way moral. He is, however, recognizably human.
When people ask for a character to be likable, what the really mean is a character that they approve of, that exemplifies their ideology or embodies their life lie. These reader's don't want real humanity. They want a plaster saint.
For all other readers, though, it is all about making a character who is recognizably human and whose frailties we can sympathize with and whose career we can follow with interest (even if with horror).
You want a character to be engaging, not necessarily likable. Breaking Bad doesn't have a likable or even sympathetic protagonist; it has a competent one and an engaging mystery. An unlikable character is a handicap, so everything else you write will have to carry the dead weight; but it is possible to turn that dead weight, that liability, into an asset.
The seven deadly words of a book are: "I do not care about these people."
Note, it's not "this person", but if a reader doesn't care about anyone in the story, they are likely to put it down and often correct to. Engagement/Caring doesn't mean like; but you should be answering the question "Why does the reader care?" often and strongly. Books that undersell often have this problem.
It doesn't have to. Even if the guy is a unlikeable and a jerk, make him human. Give him reasons for his flaws. The reader will be connected to the character at least on this level.
I recently read a detective story where the main character wasn't very likeable. Yet, the author had reasons for all of the character's flaws. The character was interesting and deep. As the story progressed, the character became more likeable as you realized why he had his flaws.
However, if you are going to take this approach realize that your readers likely won't be there for the characters. There are some stories that focus around character more than the plot. If you have an unlikeable character, I would caution against focusing on the characters too much.
I think this approach (unlikeable character who eventually redeems himself) can create a much more memorable story than if your followed the common pattern of a likeable protagonist.
In summary, I think it can definitely work, but give the character reasons for his flaws and try to have an captivating story to make up for the lack of a likeable character.
Give the reader something to connect to early on, and the protagonist doesn't even need to redeem themselves. (No reason why they shouldn't, it's just that it isn't necessary for not alienating the reader.
My favourite example is Humbert Humbert from "Lolita". The man is a paedophile. He's despicable. Yet he's so intelligent, so sharp, so engaging, you keep reading.
Another example: Satan in "Paradise Lost". His pride, his independence, his leadership, even his language, evoke not only empathy, but admiration. "The mind is its own place, and in itself / Can make a heav'n of hell, a hell of heav'n." or "Better to reign in Hell, than serve in Heaven" - it's beautiful, and easy to identify with.
Nonetheless, I find your description troubling.
a very shitty person that makes bad decisions for the wrong reasons
If your character is petty, cruel, stupid, and doesn't have any desire to change, what engaging quality does he have that I can connect to? He might have one, but if so, you haven't mentioned it. You've got to give a reader something.