If the resident had peered through the window and observed firearms already pointed in his direction by people who were clearly identified as police (there's nothing to say they were pointing at him or that they were clearly identifiable as police) it would behoove him to find out why they are there before opening a door and pointing a gun at them. That's just stupid, even if the police are in the wrong.
If the resident had peered through the window and saw random people standing on his porch already pointing guns at him, his decision to open the door was tactically stupid as well, whether he had a gun or not. The occupants of the porch, however, had not begun to beat down any door, and had only knocked on the door much like any person would do to rouse a person inside. If the people were not pointing guns at him already, he still had no reason to point a gun at them.
Florida’s justifiable use of force (Statute 776.012) indicates that…
A person is justified in using deadly force and does not have a duty to retreat if he or she reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself or another or to prevent the imminent commission of a forcible felony or under those circumstances permitted pursuant to 776.013.
776.013 “Home protection; use of deadly force; presumption of fear of death or great bodily harm”, A person is presumed to have held a reasonable fear of imminent peril of death or great bodily harm to himself or herself or another when using defensive force that is intended or likely to cause death or great bodily harm to another if:
A: the person against whom defensive force was used was in the process of unlawfully and forcefully entering, or had unlawfully and forcibly entered, a dwelling, residence, occupied vehicle, etc. and;
B: the person who uses defensive force knew or had reason to believe that an unlawful and forcible act was occurring or had occurred.
We can stop at just this point, because neither statute applies to this man’s use of force as 776.012 doesn’t apply (he was within his dwelling) and 776.013 doesn’t apply because an unlawful AND forceful entering had not occurred and was not occurring. By Florida law the resident is obliged to determine that an unlawful and forcible entry had either already occurred or was in the process of occurring.