From the description of the incident, it definitely sounded like he feared for his life. A 6 foot something guy keeps advancing on him, asking why he’s thinking of the guy’s penis. He tells the guy to leave him alone multiple times, but the guy keeps advancing. He retreats multiple times, but the guy keeps at it. He even tries knocking the phone out of the guy’s hand, but the guy keeps at it.
It definitely sounds like the guy was afraid of where this was going and tried all of the non-lethal options (retreat, tell the person to stop) before resorting to pulling out his gun. The YouTube “pranker” has nobody to blame but himself. He should have stopped when asked instead of repeatedly pressing the defendant for a YouTube “prank” video.
(I use “prank” in quotes because I don’t consider this type of thing a real prank. It’s just a guy acting like an idiot and calling it “a prank.” A real prank should leave all involved laughing when it’s revealed, not leave one person fearing for their life.)