Omar Mateen, the killer who claimed allegiance to the Islamic State, hid in a bathroom at Pulse, a gay nightclub where he had slaughtered scores of people. A wall of cinder block separated him from a team of police officers outside. For three hours early Sunday morning, crisis negotiators tried to end the siege. Then Mateen — whom police said had acted “cool and calm” during discussions — talked about killing more people. Alarmed, police placed an explosive device against the wall and detonated it. The breach failed; the hole wasn’t large enough to allow for a successful rescue. A...