On "karma" and "god"

I ended a long-term relationship recently. Which is not my point today, except that when finding out the circumstances of the break-up, people try to reassure me that karma will get him. Or that he'll regret it. Or something along those lines.

What makes people believe that karma will get someone? For every example of somebody coming to ill ends, there is another of somebody who keeps going through life as they were. And really, how does it make anything better for me if the other person has bad things happen to them? Other than hoping that other people aren't taken in so completely and hurt, why should what happens to him make any difference? Only when feeling particularly vengeful is that thought satisfying, but really only in theory. If I actually believed that my wishing something bad on him had results, realistically, it'd feel worse, not better.

And then with the concept of karma. I think that most people mean it in the sense that a person is rewarded or punished according to their deeds. Who decides on said punishing? Who decides what's good and bad?

In my mind, the usual answer of “god” is ridiculous. To any rational person who has a sense of right and wrong, god is irrelevant. If her or his existence reassures somebody when they don't have anybody else, fine.  (Although I don't see how an imaginary being really helps.)  But in terms of what will happen to you, afterlife, et cetera? If it takes the concept of heaven or hell or purgatory to help you make your moral decisions, how are you in any way a moral person? If you believe that someone who believes something different from you is going to have eternal torment, how are you a good person in any conceivable way?

If not a god or something filling a similar role, I don't see how the concept of karma can work. Unless your own good or bad feelings about what you did haunts or rewards you in some way. And if that's the answer, calling it karma, some kind of conditional destiny, is stupid.  (Also, it would imply that the person had a moral compass in the first place.)  It's in the same category of Marshall's “miracles” in How I Met Your Mother.