The law of karma is as strict, relentless, and impartial as the grosser natural laws of motion and gravity. For example, if I eat the flesh of animals even though I can live as well without it, my bad karma will force me to be born as an animal and to be slaughtered myself.
Karma is the law of cause and effect: there is a reaction for everything we do. If we throw a coin up, it will come down. If we regularly put money in the bank, our wealth will accumulate. If we drink too much, we’ll get drunk. These are natural laws of cause and effect.
Karma is not dependent on whether we choose to accept it or to reject it. It’s not under our control, something we can have made to order. It’s a law of nature, just like the law of gravity. And to change its workings is completely beyond us.