When you get down to it, Kotomine is one of the most tragic characters in the series. He spends his whole life trying to find something - anything! - that makes him happy and fulfilled. Working for the church didn't do any good. Being a devoted husband and father didn't do any good. Fighting in the Grail War didn't do any good until Gilgamesh was all "Hey... Why don't you let me show you what pleasure is?"
So Kotomine finds out what it is that makes him happy: the suffering of others, leading to a non-humorous example of "Oh My God, I am a horrible person". But he still has something to cling to: the idea that Emiya Kiritsugu is just as empty as he is. Nope! The two of them couldn't be more different. They fight and he is denied his chance to get some answers on why a person like him was born. Finally, he just says "Fuck it" and wishes for the town to burn. Ten years later, he's getting ready to start some shit in the 5th Grail War and who should show up but Emiya Kiritsugu's adopted son, Shirou. Not only that, but Kotomine realizes that Shirou is that person that he'd hoped to find, the person that was as empty as him. Unfortunately, the differences outweigh the similarities, and Kotomine is once again denied his answers. This time, though? He takes it in stride. He calmly acknowledges defeat and dies with just the hint of a smile on his face.
