I'm not very surprised about Kirk's feelings towards Klingons, either. After all, they just very recently killed his son. The film is very much about old times changing into new times, and old warriors not being able to give up their old grudges.
Nor am I, and I also don't find Kirk's behavior surprising in light of how he treated the Klingons in The Final Frontier
. The Klingons were guests aboard his ship, but there was no mention of peace talks or abolishing the Neutral Zone. Kirk knew that the Klingons were going to return to their space once the mission was over.
The circumstances are different in TUC. The Klingons are once again guests aboard the Enterprise
, but their presence signifies the beginning of the end of hostilities between the Klingon Empire and the Federation, hotilities that have lasted since before Kirk was born.
Kirk was due to stand down from active duty, though it's not clear what he was slated to do beyond that. TUC was certainly his final mission aboard the Enterprise
as its commanding officer, a final assingment in a career spanning more than four decades, including Kirk's time as an academy cadet. He'd spent the majority of that time protecting the Federation, risking his life and the lives of his crew on more than one occasion. Now, suddenly, as he's about to step aside, he's been volunteered for a mission as part of a peace envoy with the Federation's fiercest rival, a rival who's officers killed his son only years before, and everything that he has previously risked his life to protect and preserve is about to change, perhaps forever.
It's easy to deal with people if there's a known endpoint or resolution to the situation, but it's much more difficult when the people in question may bring about a change in one's own life. Kirk had never known a Federation at peace with the Klingons, and the idea of a disruption in the status quo scared him. It's not surprising that the fear he experienced drove him to say things he later regretted.
"Gorkon had to die before I understood how prejudiced I was."
In the end, Kirk did his duty, and helped to preserve the possibility of peace that Gorkon died to bring about, an action that was absolutely in character for him.