What's Bonnie's deal? Why does she hate so much Damon?

Well, let's see. There is the thing he did to Jeremy and not to mention the fact he BIT her and I think ALMOST KILLED HER. So, I think she has a valid reason to hate him.
Leave the girl alone. They'll end up bonding eventually.

-don’t read if you haven’t seen the first two episodes of season 2 yet-

To be honest, Bonnie's hate is just her holding a childish grudge against him. In season 1, Damon not only bit her and tried to kill her, but he also got her to help open the tomb with the help of her grandmother. When Grams died after the tomb opening, Bonnie began to hate all vampiros for it- with her fury directed at Damon and Stefan. She only really mistrusts Stefan, but hasn’t done anything to try and hurt him since she ‘went away’ and came back mais powerful. Instead, she started to blame Damon for absolutely everything. The vampiros escaping (when she was the one doing the spell to close the tomb again), Caroline getting hurt (when she was the one who didn’t take the spell off the device), Caroline turning (when she was the one who told Damon to give her blood)... can you see a pattern forming here? I am not saying Damon is innocent in all of this. What he did to Bonnie was wrong, but you could constantly see him trying to make amends for it. On Founders Day, he thanked her for helping him, and didn’t go and try to kill her when her help turned out to be fake. In the hospital in ‘The Return’ he gave Caroline blood, and asked if they could call a ‘truce’ between them. Every time, Bonnie has shot him down on such a thing. As much as Damon had done terrible things to Bonnie, her list of the things she’s done to him is hardly squeaky clean. She’s tried to kill, or have him killed, multiple times. I think it’s about time she got over it and left Damon alone.

well, he was a bunda to her for literally the first couple seasons until they bonded, so i can't really blame her