Well, a couple of things... if you're a Buddhist, or practising Buddhist, it is one of the central tenets of Buddhism, and is taught extensively by the Buddha. As in all things, you are left to make your own mind up, because you are encouraged to never take things at face value, but to test and examine them extensively for yourself. But let me tell you, as an in-depth, convoluted and penetrative argument - it's hard to invalidate it.
If you're not Buddhist - then the question of Karma is not of relevance to you.
But if you believe that bad things do tend to have bad consequences, because people eventually notice, presumably, you may well be of the same opinion about 'Good things'... that people notice those too. Yes?
That's Karma.
Whether you believe in it or not - it happens.
so actually, yes, you do believe in it. You just don't call it that.
It's like I said in my post. It's not a question of judging or criticising or condemning. Karma is a process, not an evaluation.