This is going to be an unpopular opinion, but as someone who has fired multiple slide/bump fire guns, more people are probably alive today because of the bump fire stocks this guy was using. True automatic fire is not particularly accurate - a lot of bullets are sprayed in a short amount of time, but they're not really going where you intend. Compared to true auto fire, bump fire is infinitely more inaccurate - you need to use one hand to 'pull' the stock forward which basically destroys any ability to meaningfully aim the weapon. It is fun as a toy at a 20 yard range, but at 250 - 300 yards that this shooter was firing, those bullets were flying in the general direction of the crowd rather than at particular targets. He managed to shoot a lot of bullets quickly, but the total lack of accuracy with that platform and at that distance very likely saved lives.
The sad reality is that if he was shooting a plain ar-15 (without a bump stock) with a moderate scope, he probably would/could have killed dozens if not hundreds more in the span of time that he was shooting. Personally, I don't really see any reason for civilian ownership of these bump stocks - but I think we're fooling ourselves if we're blaming the bump stock for increasing the number of people this man was able to kill. If anything, the opposite is true.