So when you’ve got a protagonist who suddenly stops being, well, the pro- type of tagonist and moseys on over to the an- side of life, in media that you’re consuming you can basically do one of two things. You can give up in disgust, or you can hate-read/hate-watch it to see if they will get what’s coming to them.
As a writer, you want to have written the character well enough that the audience will choose the latter.
It is pretty risky to set out writing a non-sympathetic character in the first place. Sometimes villains are so bad, with only enough traces of humanity for us to recognize ourselves in them, that we ferociously cheer for their demises, and reading a good villain death can be quite cathartic. But your main character can, should be, different. After all, that character is the one the audience is supposed to identify with.
It’s kind of annoying to read about a jerk who just wanders around getting himself and other people into trouble.
Now, a sympathetic character who turns into a jerk? Again, if done well, with careful attention to the character arc, that can be a successful story that keeps the reader hooked. You can spin it as a tragedy, or as a slip in the road before the character becomes kinder/stronger/et cetera. Most of all, it has to be plausible.
The thing is, you have to be paying attention to your characters. If that development for the worse is on purpose, then you have to show the gradual — or sudden — progression of that descent. There has to be a reason behind it. If your heart-of-gold protagonist suddenly tortures a baddie into giving crucial information, I don’t care if it’s your character or not. You either have a solid reason for why, or you get jettisoned by the reader’s disbelief. Acting out of character (or OOC, as the fandom circles term it) is the number one way to lose your audience.