You know that his life wasn’t in danger because you know it was a prank. The defender here did not have the benefit of that knowledge.
When someone nearly a foot taller with 50 more pounds of muscle gets in your face and starts accusing you of thinking about their private parts, then you get to decide if self-defense is necessary. The defender tried to get away from the aggressor multiple times and told him to stop multiple times, but the aggressor obviously kept engaging.
Do you think he should have waited until the giant started punching or kicking him before he decided to draw his weapon? Should he have tried to punch the giant instead of using the weapon he has? Do you think you could outrun someone taller and in better shape than you are?
If the defender had multiple self-defense options he might have chosen another, but all of this is arm-chair quarterbacking. We weren’t there and the person that was chose the only real defense option they had.
This situation is entirely the Goon’s fault. He should be the one facing charges. While it might be reasonable to recommend a self-defense course for the victim, my understanding is that he already followed good self-defense principles.
Edit: Also, since no one else is saying it, I will. If the victim happened to have been gay, the aggressor probably would have been charged with a hate crime. The goon was clearly accusing him of it.