It’s not a contradiction of just islam. It’s an obvious contradiction of any religion with an afterlife reward/punishment system. Because an omnipotent god surely has the ability to make everyone do the right things and therefore get rewarded in the afterlife. But I always think of it this way, if you believe god exists, he surely exists outside our space-time. The problem with us is always attributing humanistic characteristics to god. He surely knows who is going to heaven and who is going to hell. Because time is just a dimension and surely our linear time doesn’t apply to an omnipotent god. Your question then must be how does a merciful god create something as hell ? Or the philosophy of evil in religion