Whether karma exists or is a process, I want to understand who pulls the karma strings. I don't think karma is like gravity. Gravity doesn't require anybody pulling strings. Gravity simply is.
Karma, on the other hand, requires negative or positive things to occur as a result of our behaviour. Somebody or some force has to apply specific applications of intelligent power consistent with karma.
Does the pottery say to the potter "why did you make me like this, and why do you have me store fishing bait when I want to store diamonds?" Man can't tell God what to do.