Responses
He accumulated the karma because of the penalty he received from using his destiny stealing skill in his past life
Pretty much, "stealing other's fates" is a heavy karma. He took like what about 100 or so people's fate's before he regressed, but since the karma is attached to the soul it remained even though the skills did not. Depending on the level of the fate and grade determines the amount of karma he took on. We could see he earned very little karma on his record when he stole the fates of the humanoid Oni's in the first gate's secret maze.
I forgotten how he get so much karma when he didn't even kill im too busy to reread please remind me