The cop is under no legal obligation to go after the gunman although still in many departments the culture would compel him to do so.
Not sure if there is a 'Good Samaritan' law in Florida that would protect the cop if a stray round he fired struck someone other than the gunman.
You are new to this. Totality of the circumstance matters greatly. If it was a hostage situation, you might have a point. But when the murders are ongoing and not stopping, the liability flows the other way.
Watch the lawsuit based on doing nothing.
And a lawsuit isn’t that big of a deal for the cop. In all but the most insane cases like the cop shooting that unarmed dude in the back as he ran away, the city pays the judgement, not the cop.