I agree, this is a bad situation. Nothing warranted the armed encounter.
How does an "armed encounter" get justified? Let me count the ways.
1. Gregory McMichael was a 35 year law enforcement veteran that arrested Arbery for shoplifting and was aware of this being a parole violation for his gun charge from 2013. Gregory McMichael probably also knew Arbery's piece of sh*t criminal brother who had a much longer criminal record.
2. Gregory McMichael was aware of numerous burglaries in the neighborhood, and especially concerning that specific house. He said that there was video showing this guy in the house at night, and we now know that there is video of Arbery from October of 2019.
3. Travic McMichael (son of Gregory McMichael) had a handgun stolen from his truck in January. As there is a good chance that Arbery is the thief, it would be foolish to believe he did not have that gun and was possibly carrying it.
4. Gregory McMichaels said that Arbery was seen putting something in his pants during one of the night videos in which he is captured on video. McMichael presumed it was a gun, which would be what any reasonable man would suspect.
5. As the video guy filming the incident drives towards the running Arbery, he focuses his camera on a hammer that has been dropped on the road. It appears that Arbery threw it down when he saw the McMichaels blocking his path with guns.
6. Arbery was a young athletic man and presumably stronger and more dangerous than a 34 year old tub of lard and a 64 year old tub of lard. Old tubs of lard would need some sort of weapon to prevent themselves from being beaten, potentially to death.
So there you go. Half a dozen valid reasons why the McMichaels should have been armed in an encounter with Arbery.