Part of this is agenda-driven. The inaction on the part of law-enforcement organizations demonstrates that it is not enough to "call 911" and hope the police show up to protect the victims. As Michael Graham notes, the Florida situation is part of a "pattern of police cowardice" which was also apparent at the 2016 Orlando shooting and at the Newtown, Connecticut shooting. In both cases, police stood outside while gunmen worked freely inside the building in question.
Thus, if police are going to protect themselves while victims are at the mercy of gunmen, this illustrates that private gun ownership is perhaps the only reliable defense — whether in the hands of professional private security or even amateurs. Opponents of a police monopoly on gun ownership have seized upon this police failure as a helpful illustration of their position.