er, saying that "you'll never trust" an ENTIRE "race"(or culture, ethnicity, etc., whatever) because of the actions of ONE member is pretty much the definition of being racist or prejudiced.
Kirk wasn't basing this thing on just one person. He has had multiple dealings with many different Klingons over his career. It wasn't just
because of David's murder that he doesn't like them. I'd say Kirk had many reasons not to like Klingons - not only did he LOSE HIS SON because of them, but he's lost crew
to them too, and he's had many other opportunities to observe Klingon behavior.
And you will notice that Kirk did immediately regret it (in ST VI) after he said "Let them die"...