So if this shooter wasn't mentally ill imbecile who want his name in the history books, why don't you explain what he was?
We can't explain a motive because there is no evidence to suggest a motive. You don't get to insert any answer because you don't have an answer.
I am willing to grant you the fact that he likely had mental health issues which led to the shooting. However, we don't know what caused his mental health issues. I think that is what people are looking for: the underlying causes that led to possible mental health issues and the subsequent shooting. Based on what I read, the guy seemed to lead a pretty normal life, other than the obsessive gambling (which does not appear to be a reason for the shooting).
As for wanting his name in the history books, meh, who knows. I am not too sure that was his motivation. Maybe it what. But, there is no evidence to suggest such.
Thus, we can't explain what he was because we do not have sufficient evidence to "explain what he was."