At the risk of derailing the thread, I'm a karma non-believer because bad things happen to undeserving people. God makes it rain on the wicked as well as the good. My hope is that true justice exists in the eternal rather than temporal.
I'm a good person, try to be kind, law-abiding...but bad things have happened to me. However, upon careful soul-searching evaluation, I can trace nearly every instance back to something I did or didn't do. Even if it's "self-inflicted" karma.
I don't think it's a matter of "bad things happen to good people" and vice versa. However, if you are unkind or hold hatred or evil or other such malicious mischief in your heart or actions, it WILL bite you in the ass. If you are kind and pliable with benevolence in your heart toward others, it WILL reward you. Eventually. Karma 101.