Yes, I've always wondered of something happened between TFF and TUC that could explain Kirk's hatred so much. Ofcourse, a Klingon killing his son can be seen as a reason, but we've always been given reason to believe that in the 23rd century, humanity was beyond such petty behavior, hating an entire species for the action of one individual. So Kirk's attitude in TUC was a bit strange.
I don't know. The idea that, on Star Trek, future people are much more enlightened and "beyond" petty human imperfections applies more to TNG than TOS, where people were a bit rougher around the edges. Remember Stiles getting all racist on Spock in "Balance of Terror," or Kirk letting his past traumas get the better of him in "Obsession" or "Conscience of the King"? Or McCoy losing his temper every other episode?
As the show reminded us all the time, humanity was still a half-savage child race with a long way to go . . . and the crew of the Starship Enterprise were hardly supposed to be a perfect paragons and role models with no flaws or dark sides. (Heck, "The Enemy Within" demonstrated that Kirk's more primitive impulses were a big part of what made him an effective captain.)
Kirk spent his entire career fighting the Klingons, who were also responsible for the destruction of his ship and the murder of his only son. It's only human that he would hold a grudge-- and that a brief, uneasy alliance in TFF would hardly heal all his scars.
As Carol Marcus once observed, Jim Kirk was no Boy Scout . . . .