"Couldn't possibly be just that an angry, misguided and disenfranchised teen had access to an AR-15 and chose to take that anger out on those he felt had wronged him"
It's misleading to state the guy was a "teen". He is 18 and, therefore, considered an adult. Technically a teen, it's more appropriate to say "adult" when somebody has left school.
In the US, you cannot buy a gun and leave the store with it. You must pass a background check first. Guns sold at gun shows or online have the same restrictions. Guns may only be shipped to a licensed dealer and the buyer may pick them up from that dealer if the background check is complete.
What is an "assault" rifle? The definition varies quite a bit around the world. What I normally see is that any rifle that has a wood stock is a "hunting" rifle and anything with a metal or plastic stock is an "assault" rifle.
The highest shooting crimes in the US are currently in areas with the most restrictive gun laws.