How would anybody know what their current karmic balance sheet looked like? They've only got their own opinion and memory to go by, yet humans are notorious for bolstering themselves up way above a realistic estimation of themselves! At least, that is true of the majority who think they can earn merit by doing more good deeds than bad ones. All Muslims think God works that way, yet they can never know if they've done enough to tip the cosmic scales of Islamic justice to get into paradise. They have no notion of God freely forgiving them their sins so that the will be cleansed in God's view, able to come into His presence.
Further, in karma, who is the debt owed to? What higher power got such a system going and maintains it? Do you know?
As for Jesus, the trouble there is that He was utterly perfect. He never sinned, which means He wasn't paying off some karmic debt. He came to Earth to bear the punishment for all the sin ever committed, in order that God would then completely and freely cancel the debt of sin believers in Jesus had, without violating His own law that all sin must be punished. Being fully human, He fully represented humanity. Being fully God, He had the magnitude of deity needed to atone for all sin. Jesus never claimed to be a good man, a good example for us to try to follow. He claimed to be the Son of God who, alone, can lead us back to the Father.
I am in Jesus' debt - utterly. He bought me with a great price and I owe Him everything, all that I am and all that I have.