While these murderers did purchase legal guns, I hardly think they would have sat down and say "Well, I guess I'm beat" if they couldn't. The black market is LOADED with guns, as I said before, and the supply hugely overshadows the demand.
If guns were outlawed, the demand wouldn't necessarily go up. The demand for illegal guns, yes, but the supply is so unbelievably massive that the price would not skyrocket. If somebody really wanted to go kill people, he could easily obtain the firearms.