Slippery slope. What about knife collectors? Baseball bats? Military memorabilia? Shoes?
Yes, I get they are guns, but it's a fine line. Maybe the shoe collector really IS crazy and going to wreak some havoc somewhere, and the shoe collecting showed it.
What happens if they find a common trait in all serial killers that is actually a "normal" thing (you pick)? Do they start keeping an eye on all people who do this "normal" thing"? What if it's certain very specific brainwave activity? Do they make everyone be tested so we can lock up the ones who will likely be serial killers?
Seems Orwellian, but not so outrageous any more.
The vegas shooter had an arsenal, but the Sandy Hook kid had one gun. How many guns does the average mass shooter own? If very few have an arsenal, do we still consider it a "sign" of future problems?
I dunno.