Kotomine, despite being a priest (and one could question how he ever made it there--or why he would want to be there), seems to have been messed up with an inverse sense of beauty. As he says himself, he finds beautiful what most people hate, and ugly what most people admire. According to some sources I've heard, Kotomine has massive powerful spiritual healing powers because "his faith is just that strong." Got me what his faith is in, besides perhaps his own sense of beauty. He lives to bring suffering because he so long had to suffer with what other people found to be right and wrong. And, as we know, he ultimately gives his last breath trying to allow the birth of Evil because he just wants to affirm its existence, or at least witness what kind of choices an evil being would make if it had no taught sense of good and evil.

I admire his steadfastness, but his character is somewhat tragic because one wonders if he could have ever have been at peace in a world where suffering brought him joy.

Archer, on the other hand, is like Kotomine's foil. He's a 'good guy', wanting to help those in need and protect them. He's very self-sacrificial and almost always puts others before himself. He believes in what might be considered the beautiful ideal of wanting to save everyone. He tries and tries and tries to live up to that, but he ends up betraying it to fulfill it, leading him down the path of hypocrisy, self-loathing, and frankly, despair. He realizes his ideal is impossible, that it betrayed him, and that he's trapped to serve it for all eternity. His only wish was to destroy himself before he could go down that path again.

Here again, a tragic character. Even more tragic is when he's all re-inspired at the end of UBW route, he even says that he probably won't remember this encounter, which means he's still just as screwed to be bitter and numb when his job restarts itself. Poor Archer. Ganbatte.

Haven't read F/Z so don't know how much his character changed due to the curse by the grail. Dark Sakura could be one of the most screwed up characters as well if that counts.

They like to kill people and don't really have any good reason for doing so.

Trained to be a killing machine, man with no emotions. Understands what he did was wrong but still continues with that life. Kinda wish I could read F/HA so I could find more about his character.

Kotomine couldn't buy into the values everyone else lived by, even if he tried to. Archer was betrayed by the ideal he sold himself for. Sakura was kicked out of her family, used and abused by the Matous, and invaded by evil. Kohaku was used and abused by Tohno Sr and forced herself to act happy to appease her sister. Hisui refused to let herself be happy so she could empathize with Kohaku. Ciel was possessed by Roa, murdered her whole village, was experimented on but unable to die, etc, etc, etc