While I find actions similar to the OP's annoying and my CMDR would generally not behave in a such a manner, I can't say that there was anything illegitimate going on here.
More than a few times during that very CG my CMDR was attacked by CMDRs who had declared for the side he was on, either because I was the only credible opponent around, or because one or more of those CMDRs had issues with mine. Both can be perfectly legitimate reasons to attack someone.
It's fine if they chose a side, I have no objection to that at all, otherwise it's just trolling.
Assuming that only the two parties listed in the functions menu have a vested interest in the conflict is almost as dangerous as assuming that only contacts that are red before they shoot you are hostile. Both the Red Army and the Wehrmacht were frequently targeted by the partisans in the nations they had to ravage to reach each other...while such a scenario was unlikely to be the case in the OP's video, it easily could have been.
War doesn't have any rules and neither do conflict zones. The ability to declare for a side doesn't make such a declaration mandatory.
This, together with the fact that he was firing only on other commanders.
If one prioritizes targets in order of viability and threat posed, it's usually folly to put any non-CMDR above any CMDR.
I'm surprised it doesn't turn the whole CZ hostile like if you attack an NPC without picking a side.
Any undeclared interloper should probably be considered hostile by both sides, without them having to attack anyone.
next time go 1v1 instead of being a coward
War isn't a collection of duels either.