Police no longer wait for back up if they feel that there is an "active shooter" who is bent on killing everyone in the area.
Rather, the responding officer becomes the SWAT team. Officers responding even minutes later then become the back up Swat team and enter the building. The goal is to kill or capture the shooter as soon as possible and thus save as many lives as possible. Thus, the brave officer immediately entered the home, shot and killed the attacker.
Some times, however, the shooter's actions cause confusion as whether or not he is an "active shooter". For example, the Pulse attacker initially was an active shooter. He then took hostages and appeared to be willing to negotiate (at least to a degree). As a result, the arriving officers did not rush the attacker. Rather, they waited outside and called for negotiators and Swat back up. He then started murdering the hostages again.