You can be "irrationally" in fear for your life (as you put it) and be found not guilty.
For example, someone is verbally threatening you and puts his hand in his pocket in a way that looks like he is going to pull out a weapon and fulfill his threats.
As long as one could believe it may be a weapon about to be brandished, you would have a case of self defense, even if he was pulling out, say, a cell phone to record the incident.
It could have been a gun/knife/other weapon and if his action was consistent with his threats, you could use lethal force to protect yourself.
Just the fact of there being a violent physical altercation between two relatively grown men is enough for justified self defense. (I know he was 17, but have you seen kids these days?)
Especially since his head was being bashed into the ground, thats serious damage, not a mutual combat situation. People who do that aren't going to stop with a simple ass whooping.