It doesn't upset me because he didn't break any laws, and there's no evidence that he provoked or even attempted to intimidate anyone, beyond merely being present, picking up trash, cleaning up graffiti, and standing around (foolishly) with a rifle.
If you want to argue that open carry of firearms at peaceable assemblies of people seeking redress from their government shouldn't be allowed, we can have that conversation and maybe find some common ground, maybe.
If you want to argue that a 17 year old shouldn't be able to carry a firearm in public, outside of adult-supervised hunting, competition, training, or recreational shooting scenarios, we could find even more common ground.
We could also maybe discuss just how peaceful this assembly was or wasn't in the minutes/hours immediately preceding the shooting, but I suspect we'd find less common ground there.
Your hypothetical world in which "both parties have a legal right to kill the other" is also incongruent with what actually happened; three individuals were clearly the aggressors with no right to kill anyone, and one individual repeatedly attempted to disengage and leave, eventually firing only enough to stop his attackers. I think KR made a dumb decision to be there, but his trigger discipline was arguably excellent. And a jury agreed.
Context matters? It sure does. It doesn't upset me because the people who chased and assaulted him were a trio of violent felons.
- Rosenbaum (deceased), a convicted sex offender who raped five boys between the ages of 9 and 11, who wasn't exactly a model prisoner (42 infractions while incarcerated, mostly for assaulting staff). Still had an open case for bail jumping, and a pair of open domestic abuse cases.
- Huber (deceased), convicted of domestic violence, use of a dangerous weapon, strangulation, suffocation, false imprisonment ... then after release, while on probation, another conviction of domestic abuse, disorderly conduct, and battery. At least ten periods of incarceration in Wisconsin.
- Grosskreutz (minus a bicep), a felon illegally carrying a gun, previously convicted of felony burglary, criminal trespass, among other charges. Had at least one felony conviction expunged from his record.
The context here is that these three deplorable human beings went looking for a fight, and they found one. I'm totally not upset at all with these guys getting shot while assaulting someone.