If Lacy was feigning surrender, he knew the only way out of the attic was through the police. Fight and flight is a form of "bustout" seen many times before — particularly in hostage situations. "Bustouts" usually end in shootings.
We know action is always faster than reaction, which is why we utilize cover (something that stops bullets) over concealment. So if a suspect shoots at us, cover both protects us and buys us time to defend ourselves.
Years ago, my department had a veteran defensive tactics instructor, with a multiple-degree martial arts black belt. One of his favorite things was to ask anyone to try and "beat him to the punch" [literally]. No matter how many tried, no matter how good, fast, experienced they were, they never "beat him," and they got themselves punched. Same with gun draws. Action is faster than reaction.
Consequently, police need to design, and employ, tactics that effectively take away suspects' action advantage. This includes the use of cover, suspects facing away, hands up (as in "reach for the sky"), proning, kneeling, officers with multiple cover angles, lights in eyes, TASERs, chemical agents, and less-lethal.
With the St. Petersburg shootings, we'll know the details when the investigations are over, and revealed in debriefings.