In the movie Capote, why does Perry go about shooting every family member in the face? One minute he's caused the throat slitting of the father in the family. The next thing you know he's going about killing everybody. Looks like he initially didn't intend to do this. What causes him to commit all the murders?
I would say you'd probably have to read the book In Cold Blood or research what you can of Dick Hickock and Perry Smith. As it was (the crime) way back in '59 before the internet there is not a great deal of information. The book is probably the best way. Here is an excerpt of an article on this subject:
Capote explores the motive again and again within his text, eventually concluding that any real motive for the crime lays within Perry — his feelings of inadequacy, his ambiguous sexuality, and his anger at the world and at his family because of his bad childhood. Dick plays the role of true outlaw, but the impact of the killings weighs heavily on him, and his own role in the murders remains unexplained and unclear. CliffsNotes on In Cold Blood