A 27-year-old Las Vegas man drove cross-country in a BMW with an M4 automatic rifle. He arrived in Midtown Manhattan on Monday and strode cooly into an office building on Park Avenue, where he sprayed bullets into the lobby, killing a police officer and two others. He then took an elevator to the 33rd floor. Office workers barricaded the doors with desks and covered the windows as he stalked the hallways and eventually killed one more person before turning the gun on himself.

Why does this keep happening?