Here we have a man who went on a shooting spree. He was a convicted felon, which means that he could not legally possess a firearm. He had a history of mental illness, also meaning that he could not possess a firearm. Still, he got one anyway.
There are those who say that making guns illegal would keep everyone, innocent and criminal, from obtaining firearms, and thus committing crimes. To those people, I would point out that he had also been convicted of cocaine possession. Cocaine is illegal, and has been for decades, yet no one who wants to buy cocaine has any problems getting it, as this Federal Judge who was just arrested for cocaine, marijuana, hookers, and illegal firearms possession proves.