You can here him saying "I'm military... I'm on your side"
I think the Sheriff's deputy was telling him "Don't get up"
If true
exactly why the command should be Stay down, not Dont get up.
Police orders in this sort of situation should be in the of "Do X", not "Don't do Y". And the victim told the officer he was in the military, so that's how the officer was supposed to know. And when the victim responded to whatever the officer said, by saying "I'm gonna get up", that should have have been a tip-off to the officer, who for the sake of argument we'll assume had said "Don't get up", that he needed to immediately rephrase his order as "Stay down", not that he needed to fire on the guy.
That would make a significant difference. The way it looks at first the cop is telling him to "get up", and then proceeds to shoot.
Not to blame the victim here, but the only smart reaction to a cop with a drawn gun is to be quiet, wait for, and follow instructions - approaching is never a good idea, nor is arguing.