Karma is a just world myth. Karma guarantees justice in every case. No one need do anything to achieve it. Justice just manifests without any need for intervention, let alone with the need to inflict suffering on anyone.
But of course no one believes in karma in this pure sense. Everyone backs themselves as "good" (fair, just, etc) and as an agent for good. We constantly intervene to address issues of fairness. And we feel fully justified in doing so.
In practice, belief in karma reflects the thought: "It's not fair and it ought to be!" Life is not fair. It never is.
"The brightest ones of all, early in October fall..."I do not believe in karma. I do not believe in guaranteed justice or deserved suffering. Rather, I believe that no one deserves to suffer, not even those who cause suffering; and that justice is manifest in how we treat each other or not at all.
This is the principle of ahiṃsa or "do no harm".
Of course the implications of this are complex. But morality always is, if you take it seriously. And, of course, I am far from perfect. Still, this is what I believe and why I don't believe in karma.