The photographer's lament has been criticized elsewhere and the critics deftly filleted by Uncle. Having read the photog's account, the critics are alllll wet; armed police officers were on the scene, hiding, perhaps fearing to make it worse. Had one armed citizen been able to drop one of the jerks murdering people or even made a try,* it is probable they'd've taken their shot, too: bravery is easier to imitate than originate.
Soapbox ('cos I'm not Breda and not succinct): someday it may be my turn, or yours. While I've scant respect for altruism, in a situation where one encounters villains tramping through killing folk as the whim strikes, there is nothing to lose; odds are you're dead anyway. Me, I'm taking along an honor guard of civilization's enemies. I recommend the same to you. If we're serious about stopping this sort of thing, we must make the cost of it too high for even barbarians to bear by stopping them in the act, as swiftly and violently as we can. Police and the various TLA-type agencies are good for picking up after, at containment when possible, even at ferreting out malefactors before they strike but in the moment, it's like a fire, like a car wreck, like flood and tornado: it's up to everyone to do whatever they can.
Are you prey or are you human? Better answer it now and make your preparations accordingly.
* It's easy and catchy to call 'em "terrorists" but the term's become dilute and worn from overuse.