Another senseless act that saddens us all, and our thoughts go out those affected.
The argument that law-abiding citizens should be allowed to own guns seems rather ironic given that this shooter had no criminal record making him a law-abiding citizen. He will remain on record as a law-abiding citizen, as well, as there will be no conviction in this case. 
Many others cases like this exist in U.S. history. The gun lobbyist's 'law-abiding citizen' argument needs to be challenged.