He had so many issues with coworkers over the years at many jobs, making false accusations that they were out to get him and calling him names. @ one point, his employer directed him to seek medical attention or he would be terminated. Once, he was fired and had to be removed from the premises by the police.
He claimed Jehovah told him to do it.
Yes, it could be just anger issues. But...there is a legitimate suspicion that this was a mental health issue. Consider this: paranoia (everyone is making racist comments about him), command hallucinations (Jehovah speaking to him), delusions of grandeur (saying those other mass murderers are "his boys"), bizarre behavior at work (former employers state this) and aggression characteristic of several common mental illnesses. It's well within the realm of possibility.
Now we will probably never know for sure, since we can't have him diagnosed now.