The reason why you do not touch fire is because you know that it will cause you to suffer. Likewise, if you truly understand karma, you will not commit a single negative action, because unless that negative karma is purified, you know that it will eventually ripen into suffering.

You might forget this natural process, or you might not believe in it, because the ripening does not always happen immediately. But your karma will follow you like your shadow, that gets closer and closer without you realising, until you are eventually touched by it. Please, I urge you to always remember this.

-- Chamtrul Rinpoche