Perhaps we should also discuss the idea that karma is just a primitive way of accepting the actual determinism of the universe: What you do has effects which happen inexorably. So, in other words, if karma is real, then that is one more blow against the mistaken notion of free will.
I do not believe that "karma" exists. I think that everything that we experience has a partial degree of randomness to it. Sure, there are certain things we can more or less control once we pick up particular patterns of randomness. However, when something bad happens, it is because we interpret it as "good" or "bad". What I am getting at is, you did not likely stub your toe because you cut someone off in a traffic jam last week. On the other hand, you did not win the lottery because you were nice yesterday and let everyone on a busy highway merge into your lane. You stubbed your toe and won the lottery because they were random events. Society has decided that one of these events is "good" while the other is "bad". Next time you have dice, role them. Let's say, every time you role a 1,2 equate that with "negative" or "bad". Every time you role a 3,4 equate that with "good". Finally, every time you role a 5,6 that is "neutral". Role this dice 100 times, and record your results. You will see that there is total randomness. If you act well, or poorly, it will not make a difference. You may have social issues, but as far as the issues you have no control over are concerned, karma is man-made. It was created to place boundaries on society and try to discourage poor behaviour.
What did a baby with brain cancer possibly do to deserve that? Karma is nonsense.
No evidence anywhere that karma exists (and some evidence it doesnt (at least for those who believe it occurs only during the life span as a humabon Earth). Its hard to imagine why a small child should be subjected to abuse, starvation or a slow death by HIV...or someone forced into lifelong degrading servitude.
Kibd of karmas
Karma means different things in different religion. Taoist karma is closest to what westerners think of karma (the ways of nature balance the light and the dark) though its still not a "what you do comes back to you". Some bhuddist schools have a weaker version of Karma (more of a middle path is best than a balance sheet of deeds and consequences) but it is spread over mantly lifetimes with the objective of reaching or getting as close as possible to transcending the cycle. In Hinduism karma is not what westerner think of as karma but is a."your way" or "your deeds" and doesnt infer that youll get whats coming to you but is instead part of a six fold set of individual actions including dharma etc that leads to good things.
Psychological and enviromental consequences
While I agree that one can create their own hell by questionable actions (retribution, guilt, decline into a criminal violent world, toxic mental habits)...it has nothing to do with physics nor nature (beyond human psychology) nor the universe. People do terrible thigs and suffer depression and shame and hate and loss. Few escape at least some of these problems. Yet some people do horrible things yet avoid a personal hell and one can do mostly praiseworthy deeds yet still suffer immensely.
Does any karma exist?