And I have said many times there is no conflict between science and faith or belief. What I said, when you think you doing good, you can also do bad. Karma as I know buddhist practice, ties them both physically and verbally from doing what is best for them, to be a better person. Karma belief like buddhism practice was the question, not what a "freethinker" might practice or believe.
Karma can be you, your, guilt, consensus, or it can be universal in form where many people wish bad things to happen, or opposite.
What generates such energy I do not want to speculate in.
Unfortunate there is no proof neither prayers or karma exists, or work, even miracles happens all over the world on daily basis. Bear with me, I just wrote this on the run ????