There's a lot of comments coming out now that both he and his mother suffered from some kind of terrible chronic back pain, and he himself had some kind of surgery on his back and that he and his mother both had big problems with UHC in this regard. It might still be a bit early to say if these stories are correct or not, but if they are, it would provide ample motive for the crime. It's a very small step from seeing your mother suffer greatly (and you too) because of insurance company malfeasance to wanting to kill the guy in charge.

Also, this is a quibble, but I don't think he saw the CEO "die in agony": he fired his 3 shots (with some difficulty because of the particular way the gun and suppressor worked), and then took off to get his bicycle. The CEO later died in the hospital.