Karma is basically real
Your karma sticks with you not because of cosmic repercussions, but because people are consistent and because we get stuck in our patterns and usual ways of acting. Hurting someone once means you're more likely to do it again. Hurting more people means others will generally trust you less and might want revenge. You then actually experience more people out in the world being less trustworthy and hurting you more, and (sort of rightfully) determine that they don't deserve your trust. Your acting distrustfully created a less trustworthy world around your. More simply, karma is basically real.
You are one half of every interaction you'll ever have. This drastically affects your theory of how people tend to behave. If one person's an asshole, they're an asshole. If everyone's an asshole, you're an asshole.
We have an accumulation of habits and tendencies that may as well be called karma. Not only will others create karma for you, as they will want to reciprocate both your positive and negative behaviors, but you will also get stuck in your own habits that influence how people generally treat you.
So -> Help others to help yourself: Enlightened Altruism.
"Every Action is a vote for the kind of person you want to become" - Atomic Habits
People are consistent and reciprocal, they more you hurt/help them, the more they will want to hurt/help you.