This was a question that came up several times during my classes in Hong Kong - even in those I did with school kids. What happens when you create a main character who's not very likeable? Does it matter? Isn't it interesting to the reader to find out why he/she is unlikeable? Wouldn't the fact that the character changes and becomes OK by the end of the story make the reader keep reading?
Mostly, no. There are always exceptions, and the one that everyone tends to quote is the guy in American Psycho. Because lots of people read that book, or said they did. But did they read it to find out what happened to the main character, whether he came good? Or because it was so violent and disgusting that they were waiting for him to get his come-uppance? Some people read it because it was cool to say you had. I've never heard of anyone, even reviewers, who said they liked it, and liked the main character.
Like is probably a misleading word. What we usually talk about is empathy - we feel something for the mc, perhaps pity or some kind of identification, and we grow to care about them. But that usually only happens if the writer gives us something in the first few pages to latch onto. Something hopeful. Something that suggests this character has another side that we might like if we're let into it a bit more. We keep reading because we hope the character will redeem him/herself, show they aren't so bad, show they can change, show that they will come to understand the world and themselves a little more. (OK, my him/her and them is a bit mixed - please ignore it.)
The most common reaction to an unlikeable main character is to stop reading. Who cares if he/she dies? Wins through? Changes on Page 299? If we're up to Page 20 and the character is awful or stupid or apathetic or depressing, we stop. Plenty more books out there.
The villain, of course, is a different animal altogether that I've talked about here before. This issue came up because of a book I've just read. The Watchman by Robert Crais. If you're a Crais fan, you'll know that his detective is Elvis Cole, whose sidekick is Joe Pike. Inscrutable, iron-faced, unfeeling Pike. Now Pike gets a book all of his own, with Cole as the back-up. If you want to read something where the main character is unemotional, cold-blooded, and acts like a machine, and then see how the writer gradually unpeels him, little by little, to reveal his vulnerable side, this is the book for you.
Crais never overdoes it. All the way through, Pike remains the consummate soldier of fortune, able to kill without compunction when required. Yet every so often, we see a little crack of light, and even though most readers probably won't finish the book "liking" Pike, I think they'll understand him better and feel that empathy I mentioned.