ashes ashes we all fall down

Today a man shot both of his parents, his cat, his dog, and then himself. They say it's possible he had a complete mental breakdown. I wasn't on scene but I was told it happened over a course of a couple of days. The man first shot his parents then lived in the house for a couple of days before doing himself in. I know it's tragic that his parents were shot, but what compels a man to kill his cat and then his dog? He had fur on his hand indicating that he had pet the dog before he killed him. What was going on through this mans head while this was going on? I can tell you the last thing that went through his mind....it was a bullet.