Karma is a psychological idea we all have in our brains.
Good and bad things happen to us everyday, all the time.
The ony reason we think Karma could dxist, is because when we make a good/bad choice, nothing will come of it.
The only way something would come of it, was if you believed that that something that has just happened to you, good or bad, is due to your previous action. We only take note of these good/bad things that happen all the time when they are after you've done something else that was good/bad. We automatically associate anything positive that happens to us when we've done something positive prior to. Same goes for a negative act.
This is why Karma doesn't exist, and that it will only exist if we believe it exists.