That's not muzzle flash, just standard nav lights.

If there were a helicopter flying with guns the first questions would be why?, the second issue is all scenarios like that require flash suppression for the very reason of reducing the chance of detection.

A military based mission with a pickup on a rooftop in a civilian area would be done by two helicopters at a minimum.
One for the pick up, the other flying overwatch to ensure the landing area is clear and to provide fire support/area suppression.
And all the nav lights would be off and the blades feathered to reduce the noise profile.

A non military group doing this probably would operate in a different manner, but even then, firing from an airborne platform is challenging at best and in the context of picking someone up from a rooftop the only reason to fire from it would be if the pickup spot had someone they didn't like in it.

Vegas is the kind of place I would expect multiple civilian aircraft to be flying at any given moment, to jump to the conclusion the thing in the video is anything but a tour heli is to make a leap beyond a sensible boundary.

If it were a black op, using an air pickup would increase the likelyhood of mission failure, due to being seen, mechanical failure and also time in theater - picking up with a bird can take longer than a few pre-prepared ground options and usually is used when the ground options are not available.
In the context of the obvious conspiracy around the whole shooting, extraction via ground operations would make the most sense as it's pretty easy to get about unnoticed in densely populated areas.