Thank you for correctly calling him a murderer, and thank you for correctly not using his name. I will never say the names of these people. They should be executed and buried in unmarked graves.
It does appear that in this case, the higher capacity of the 5-7 played a role. The specific details have not yet been released, but it appears that he fired a LOT without reloading and did a LOT of damage. He had a higher hit and kill ratio than most other similar events.
The modern FN 5.7 round has slightly more energy from a pistol-length barrel than a .22 magnum has from a rifle-length barrel. We have participated in heated discussions in whether or not this is necessary or beneficial. I have decided that I would not use this for a carry gun, it's very big, and I am not at all convinced it will do more damage than my .45. I suppose if one had a specific need to have a gun to fire into other cars, which is possible in today's society, it would be good to have, but if I were in that situation, I would prefer to be using a rifle anyway.
Yes, look for a resurgence from the antis about this evil black rifle in pistol form.