To feel better. Whenever someone is stressed out, angry, frustrated, jealous, envious, and they blame it on anything or anyone other than themselves they instinctively want to retaliate, hurt, and in some instances silence/kill others. However to kill someone a person either doesn't think about the consequences or they don't care about them. All that matters to them (in the moment) is getting rid of their enemy. Murder for hire on the other hand is all about making money. If one offers enough money to kill another someone is willing to kill for them.
I would say it's "the lack of money" or "the worshiping of money" that is the root of all evil. Crime is always highest in poor neighborhoods. Greediness is also a negative trait. It's people and their motives that give money a bad name.