What's the reason murderers do this?

THEY ARE USUALLY CALL "CRIME OF PION". That pertain to people who kill spouses or lovers out of jealousy which is the usual reason. If one guy suspect her wife is meeting another guy so same goes to lover. The simply reason guys do not like to be taken over by deceit and fraud by the very people they love most. They feel betrayed. Therefore the only solution they can only think of is none other than to kill this people.