Advocates for gun control frequently point to the UK's gun ban and its alleged success in preventing mass murder. But in terms of mass murder victims in ratio to the overall population, it seems mass killers in the UK have been quite competitive with those in the US in 2017, if not out stripping the US in terms of victim to population ratio, and they haven't needed firearms to do so. Thus far in the UK in 2017, a bomb was used to kill 23 people and injure another 250, while two vehicle/knife attacks took another 14 lives and injured more than 90.
Other mass murders with death tolls vastly outstripping events in Vegas:
One guy with a small bottle of paint thinner killed nearly 200 people on a South Korean subway in 2003. One guy with a jar of gasoline killed 86 people at the Happy Land club in NYC in 1990. It's been speculated a disgruntled gambler set fire to the Winecoff Hotel in downtown Atlanta in December 1946. That fire killed 119.
Yes, we'll see conspiracy theories rising out of the Vegas shooting. Most will be rooted in people trying to preserve their normalcy bias that just one person couldn't commit such a horrific act.
But the fact is, yes, it is possible for one person to do something this horrific. And unless there's real evidence to the contrary that emerges later, the Vegas massacre appears to be a solo operation.