Without trying to come off as a paranoiac, my genuine suspicion is that banning these 'scary rifles' that, by your own admission, you didn't know much about until you performed a LOT more research than the average person is likely to ever do, that it's the first step in eventually banning more and more categories of guns until eventually, they're all banned.

The other points I'd like to make that I feel are somewhat relevant is that you should very much read up on DC v Heller, which is the most recent Supreme Court decision on the matter (except for Heller II) that specifies weapons "in common use" as a protected class of weapon from government bans.

I would go further to suggest that I honestly believe (though I cannot substantiate) that the AR15 is able to be categorized as 'evil' because of its use in some of these high profile, but otherwise unlikely mass shootings, but what I really think is that they're most likely being used because they are just so popular. The AR15 is the most popular rifle being sold in the United States, and has been since 1994.