*** SPOILER ALERT SPOILER ALERT***
Paltrow is a relatively pure woman, virginal, and attractive for that. Spacey decapitates her.
Spacey knows Pitt is going to view his wife's severed head.
Spacey knows that Pitt will lose his mind temporarily and kill him despite that Pitt is well-aware that he will go prison.
So a demented f*ck like Spacey wins.