why is he still caring about his master? he was an evil bastard that let you sacrifice your soul for eternal torment, doesn't matter if he regretted it or was against it he fully knowingly and willingly went along with it.
@Doomer he's like a father to him, no matter if he make MC become sacrifice or some shit, even at first he hate his master from bottom his heart,
but after seeing his diary of how he feel about MC, he know he still love and care about his master since he basically the only 'family' he has...