I'm a dog lover, what can I say? And the fact that he was guarding the grave of his master just made we want to kill him less. The music is also really fantastic for this particular boss fight. Man, and that limp animation when he gets low on health, you really feel like you are putting the poor beast out of his misery.
It took me back to Shadow of the Colossus when each time I kill one of the Colossi, I felt like a monster.
So guys, have you ever really, really not enjoyed killing a specific character in a game???