Based God said:
Karma is revenge. It's the world getting revenge on bad people. So in turn, you do believe on in karma.
Karma is karma.
Revenge is revenge.
In my opinion, it's rediculous to call karma karma. Obviosl if you kill someone their sibling is going to want revenge. That's not karma. What about all the I innocent people that die and the murderers get away with it? You either get away with it or you don't. Depending how good you are at it. Just because your nice to people doesn't mean they'll be nice back. Etc, etc. I believe in common sense/coicendence's.
What if you shoot someone and you got shot back one year later.
You go back in time and don't shoot anyone but stil get shot one year later. No matter what you do you get shot. That's a coicendence. You may think you got shot because you did it too but in reality that doesn't make sense. If you didn't shoot anyone, and got shot, you wouldn't bat an eye. You would never think about karma. Something bad happeneds to someone every single year. Doesn't have to be karma.
I'm sadly not too good at explaining it so I googled it and found something similar here:
http://www.selfgrowth.com/articles/Ellis11.html
Edit:
I'm on mobile and I'm sorry for any grammar errors /;