Karma is what keeps me in line. I'm scared of Karma, I know that bad things will come back to bite me in the ass every time.
Not everyone believes in Karma and I get that, some people believe in free will and that's fine too. My mom is Catholic and therefor doesn't believe in Karma however, she does believe that "what goes around, comes around," which to me is the same thing.
I think there are lots of ways we, as humans, justify having to be good. I mean laws do that too but there are no laws about being an asshole. Most people realize that there are repercussions to being a jerk and one of those is that it will come back around and screw you in the end.
When I see Karma at work it makes me giddy. Deep down I'm a vengeful person but I never act on it, I let nature do that for me and it always makes me smile when I witness it.
Mean people suck!