It's completely reasonable that people feel the need to over analysis and dissect the killer's behavior, hobbies, and persona in an attempt to correlate something to the tragedy. The methodology isn't reasonable because it's unlikely to source a cause, but I understand why people do it.
Regardless of American's obsessive culture and protection of firearms, tragic acts like this are dramatic statistical blips of conscious behavior. And naturally we as a culture and species would like to identify why the person acted the way they did, in part to give catharsis to the act itself (even if the tragedy remains), and also to prevent the act from reoccurring.
"He just flipped / just a madman with a gun" isn't a satisfying conclusion, largely because it's not true. It hand waves exploration of what it means to be a "madman", why people lose grip of pacifistic culture, and why some lash out with such militant, targeted violence and destruction. Especially in cases where feverish ideological warfare doesn't appear to be part of the reasoning.
Unfortunately until we've a more developed and measured understanding of human behavioral psychology we're kinda at our wits end, especially with outliers like seemingly random mass shootings, and we end up having people latch on to absurd reasons for why the killings happened.
