S P O I L E R!!!!!!!!!!
To the person who was looking for motivation for Butch to go back into
the basement and save Marcallus (Denis I think?)...
I thought Butch was simply a person of honour (twisted sense of it but
honour notherless). Now I realize this sounds quite trite but bear with
me on this. There was a lot of care taken to show Butch's reaction to
his wife (?) forgetting his Father's watch. He explodes violently but
instead of taking it out on her he collects himself and forgives her, "I
guess I didn't make it clear enough how important it was to me."
For me, it is this scene that sets Butch up as, basically, a good
person. Being a good person, he couldn't just leave Marcallus in the
basement...no matter what their past. This is not to say that he might
not have killed MArcallus anyway if the answer to the question "what
now?" was not the favorable one Butch received.