The only “more” carnage would the terrorists being taken out on day one by arme citizens. Thus saving the lives of those hostages taken a day later.
I have a clamp-mount in my trunk holding a SS Mini-14 just for the unfortunate occasion this may happen... leftover habit thing from living in remote rural areas in my younger days. You know, next to the jumper cables, fold-up shovel and blanket. I hope it stays there, but I’m not going to be a helpless French policeman if something does happen.
And armed police.
Or it would not have happened at all because the cowards would not even try. The terrorists only act in "No Gun" zones.