1) Thrown popcorn does not justify use of deadly force, even by a cop. You may only use deadly force if you feel your life is in danger.
2) The moment the ex-cop drew his weapon, he justified deadly force being used ON HIM. Keep in mind this is Florida, and there were likely other armed people in the theater. Once a cop retires, then he's just another citizen, and his past occupation becomes irrelevant in the determination of whether deadly force by him was justified.
That said, I'm waiting for the police and prosecutors to close ranks and try to figure out a way to get him off.
They already are. They had the perp change into a white jumpsuit at the scene so they could take his clothes "for evidence" - yeah, that's really normal. Wonder what those clothes will come back with now? Also, he's dressed in white, a kneejerk emotional color of innocence in America. In addition, the cops cuffed him in the front instead of the back, and then a single cop escorted him out to the police car by walking - in front - of the perp, showing no concern for safety. Then when the chief and his deputies came out for the presser, they were all smiling as if, hey, no big deal.
Oh yeah, the fix is already in.