Of course I don't agree with this. I once went in to the Pawn shop and bought 5 semi automatic 22 caliber weapons in November. They were Christmas gifts for my nieces and nephews that year.
I bought 2 rifles at one time as gifts for my brother and nephew's birthdays.
Plus, I am not so sure that the killer in question bought his guns at or near the same time so as to raise any flags.
How many guns of what type constitute an arsenal? I have 23 or 24 guns right now. When I was younger and didn't have a wife and kid to support I might have as many as 50. The guys who installed my AC back in the mid 90's noticed all the weaponry in my house then and asked at the local store if the guy who lived at the end of the road in the woods was OK. The store owner laughed when he found out they were talking about my guns and said, "Yea, he's OK, he just likes to shoot."