I've seen some legal analysis (
here) that suggests that both Arbery and the Michaels may have been justified in self-defense, owing to the mistaken beliefs on both their parts.
Arbery, because he feared for his life from random strangers stopping him with guns. Even if they had no plans to harm him, he was put in fear by their actions.
The Michaels, because they had no aims to harm Arbery (or, in a less generous telling, IF they can prevail in their claims that they had no such aims...), and his actions in attacking them (even justified!) put them in fear of their life. They wrongly believed they had citizens arrest powers, and while that belief was wrong, based on that belief, they were justified.
Honestly, I have no idea if this legal analysis is correct, but even if they ultimately prevail in a self-defense claim, it's a major mess. It shows the danger of ASSUMPTIONS on the part of both parties.
Just a reminder: don't try to exercise police powers, especially with openly carried firearms, unless you are actually the police.