When mental illness was first thought of in Merry Olde England, as a defense, the standard for that was expressed as pertaining to someone who "hath no more reason than a wilde beast." That was a pretty high bar.

The Las Vegas shooter demonstrated plenty of "reasoning" capacity. He quietly got those weapons into a hotel room that he rented for the purpose of shooting out it's window. He cooly took aim at the crowd assembled for the concert. He presented at the hotel checkin desk as a lucid individual. This was not a person in a hysterical state of mind. This was a man conducting himself in a calm, cool, deliberate maner. Stephen Paddock would not have met the standard for an insanity defense in 16th century Britain.

That begs the question as to "Why?" His father was a convicted bank robber, wanted by the FBI, after escaping prison. But, according to family, his father didn't raise him, so the passing on of poor moral values from the father doesn't seem real likely. It is tempting to speculate that the father passed on genes coding for immorality. No one knows if that is possible. Like his father, he was highly intelligent. Current thinking argues that genetics is largely responsible for intellectual capacity. (which seems to have no correlation with moral rectitude.)

Wikipedia reports that law enforcement had reason to believe that Paddock had some financial setbacks that caused him to be despondent. Supposedly, he was drinking excessively and may have been abusing Valium. I don't know that that constitutes "mental illness." Even if the argument is made that he was clinically depressed, few depressed individuals go around shooting into crowds of strangers.

I think a key is to be found in his high stakes gambling. He wasn't experiencing what most of us would call financial hardship. But that is relative to a person's expectation. He was used to having a good deal of money. I think it's safe to say that compulsive gamblers live in hope of making a real killing at their gaming activity. There are reasons why various religious traditions forbid, or, at least, discourage gambling, as being injurious to the soul. It's certainly not an activity that elevates the human mind. Maybe the moral condition of Stephen Paddock was initially not that he loved evil, but that he failed to develop an interest in much that could be called "good." (The absence of fathering could contribute to that.) Maybe all evil needs, sometimes, is a void.