They may be mentally ill and therefore unable to apply morality in a way that is generally accepted. They may not understand that what they are doing is bad - or there definition of good and bad does not follow the accepted norm.
They may be religious and follow rules or instructions that contradict the golden rule. As Steven Weinberg said “With or without religion, good people can behave well and bad people can do evil; but for good people to do evil - that takes religion.”.
They may be acting under some duress. For example they may be forced to do something bad with the threat of something bad happening to them if they do not. A similar effect can occur when people are strongly indoctrinated with some idea - for example the effect that the Nazis had on the people of Germany.
I am sure there are more reasons I could think of if I thought about it for long enough. The point being that people do bad things for all sorts of reasons.
