Actually, he caused Walt's lung cancer. This little fly bastard took a small tumor out of some person a few miles away and Walt accidentally inhaled the fly into his lungs. During his sleep, the fly left the tumor there to grow in Walt's lungs, so the fly escaped Walt later that night. Every time Walt sees this fly, the fly is reminding him who the real bad guy is.
After Walt dies in the series finale (I can only imagine him dying, he has so little time left), the fly stops by Walt's corpse and takes a small bit of the cancer, passes it on to other flies. The world is fucked. I am going to kill every goddamn fly I see now.
His original intent for making meth was for his family, right? Wrong. He wanted to scope out every nearby place where he could find this fly. No wonder he is so crazy, he wants to kill the bastard that gave him this lung cancer in the first place. After seeing the fly on the smoke alarm light, he just gave up and decided to make making meth a recreational sport.