If I had to go out on a limb and guess the missing context of this video, I would assume it was recorded in Brazil, and we are looking at an off duty cop and his girl getting assaulted by two armed criminals outside of their apartment door.

The wannabe predators quickly turn into the prey as their target produces his own handgun on which he is proficiently trained, and lands several rounds into one of the attackers. The likely mortally wounded man collapses on the stairway while his accomplice rapidly exfiltrates from the area, hopefully never returning to his life of crime.