I think that nothing worths to kill someone, not even money. Those who do that kind of things should be killed too but only after they have been tortured hard, because just kill someone who has taken other people's life isn't enough in my point of view.
I do believe that greed is a strong motivation to commit criminal acts, including murder. I do not think that it is the only reason why people commit horrible acts upon their fellow humans. I think jealousy is also a very strong motive.