Dragon God - Demon Souls...I mean he's a angry dude who wants nothing but your death, but yeah, when you pin him down with two harpoons and you need those last few hits to finish him off and he just sits there wincing in pain, I feel bad.
Though I think there is a game breaking bug that if you kill him as he kills you, you will be unable to finish the game as it won't register the kill and the final sequence won't be initiated by the maiden in black. So I guess he can potentially get some form of payback.
As others have said - Sif is pretty rough especially if its after you do the DLC.