This argument certainly seems to have some logical merit. But the facts say otherwise, and indeed, even in places in which there were armed security - such as outside the Pulse Club in Orlando - the murderous devastation ensued.
Perhaps, if we had a world in which every person had a weapon, and carried that weapon just as we all carry our smartphones - that is, everywhere and all the time - a good person with a gun might be in the right place at the right time and stop the killer. But to get to that point of universal armament and carrying, we would have to have vastly more guns in circulation than there are right now, and this pitches us right into the other side of this argument, which is: how can we better control who is able to get a gun, with the goal of making sure a terrorist, psycho, or hater isn't able to obtain a weapon?
Without such better control, a move towards more not less armament in the populace would inescapably lead to more not fewer guns in the hands of killers. That's basic math and logic. So why not adopt better standards of who can and who cannot legally purchase a gun right now?
Given how well we've done in regulating who can get a driver's license, putting in stronger regulations about who can purchase a gun shouldn't be that difficult. It's the logical first step to a safer society, whatever our ultimate views about the availability of guns in our country.