It seems that in the Western world, people seem to tie up the idea of karma with sin and judgment all meted out by some divine being or force. Some branches of Hinduism look at karma this way but overall they do not, nor does Buddhism. Karma translates as 'deed' or 'act.' It refers to the principle of cause and effect. As has been said, you reap what you sow. Your actions will determine how others will treat you. Your actions now will determine how your future plays out. The actual 'fruit' or 'result' of one's actions is actually called vipaka.

Cause and effect. If I were to develop lung cancer due to being a smoker:smoking is my karma(cause) and lung cancer is my vipaka(effect). It's not a punishment, it's a natural consequence.

We already have a couple supreme forces to mete out justice...God and Santa Claus. Why turn karma into another?