Most Helpful Guy
Most people have a conscious. Those that don't, which is as many as 4% of the population, are considered sociopaths. There is a really interesting book about it called "The Sociopath Next Door"
It's one of my favorite nonfiction books, and reading it really opened up my eyes to how some people simply don't have a conscious. They will do whatever they can get away with and not feel any guilt. I highly recommend it. It will answer this question (and dozens more) better than anyone on this site can.