He was on trial in 4, possibly on condition of him being shipped to Kronos for further "trial" if the Federation had found him guilty. He was > < close to starting another war with the Empire with the death of Kruge and the theft of a Bird of Prey still belonging to them. He took it all with a remarkable calm, not yelling at the Klingon deligate, calling him a liar etc
5, he was taken aboard a Klingon ship that fired on the Enterprise for sport, 6Kirk would have been foaming at the mouth, 4/5Kirk took it in stride again, no animosity right after his son is killed by them?
Yet in 6 he can't stop himself making remarks advocating genocide by deliberate negligence? that's one hell of a step up from 'no hard feelings'.