Here's a story outline we've been seeing a lot lately (so don't worry, I'm not talking about any one story):
Protagonist is a truly horrible person. We see a whole bunch of examples of how horrible they are, over and over. In the end, a transformative experience makes them see the error of their ways and they become a good person.
There's certainly nothing wrong with the basic idea of a redemption plot. But the more time you spend in the first three-quarters of your story showing me how awful your protagonist is, the less interest I'll have in seeing the character redeemed. I don't want to read 4000 words about someone who's the embodiment of pure evil (or even pure obnoxiousness) just to get to the 1000-word ending in which they reform. Also, the more over-the-top dreadful a character is, the more likely I am to think "I bet the author is just setting up this awful character to be reformed in the end." (Note: this doesn't mean it's a good idea to set up the awfulness and then not have the protagonist reform in the end.)
One technique is to give the reader some reason to like or be interested in the protagonist despite their being a bad person. Make us want to see the character redeemed.
Another technique is to make the transformation gradual. Epiphany is all well and good, but few people undergo radical personality changes overnight. In A Christmas Carol, we see a sympathetic side of Scrooge even during the first spirit sequence. (Though I haven't actually read the whole book, so I'm not certain that's a good example.)
Another is to put more in the story than just a sequence of demonstrations of how awful the character is. Give us other characters to be interested in (but don't make them two-dimensional either). Make the plot about more than just redeeming the protagonist. Make the prose something we'll want to keep reading regardless of anything else.
Another thing to do is just reduce the number and degree of the examples of awfulness. After a protagonist has kicked a kitten, we really don't need to see much more to convince us this is a Bad Person.
(Of course, in a humorous story things are different; if you're aiming for comic effect, then over-the-top Badness can be good, as long as it's clear that it's meant to be funny. But I'm not really talking about humor here. And as a matter of personal taste, I don't usually find it funny when people are nasty to each other; I know plenty of people do find it funny, but it doesn't often fit my sense of humor.)
I think the main thing to keep in mind is that if you make your protagonist too unrelentingly annoying for too long, the reader probably won't think "What a great portrayal of an annoying character!" but rather "This story is annoying!"