Sometimes, these "bad things" that happens to you after you said/did something "bad" to someone else, are all based on your subconcious mind or as people call it conscience.
You feel bad about it and it eats up on you, and when something bad happens, you instantly think it's Karma. And also how you have been taught on how Karma works plays a part in it.

But I do believe in Karma myself, I'd like to believe in it. For the world is insanely unjust and unfair and it's nice to have a hope that perhaps somewhere, there is a fair judge to prosecute those evil bastards