At the time of the incident, Georgia law allowed a citizen's arrest provided there was reasonable suspicion a felony had been committed; in GA, criminal trespass is a misdemeanor.
Arbery was a dirt bag, but their, "reasonable suspicion," that a felony had been committed basically boiled down to, "You ain't from around here, boy."
That is *YOUR* prejudice speaking. When the thefts first occurred at that house, the owner sent them video footage of the perpetrator. Even the prosecutor admitted at trial that all those videos of Arbery rummaging through the house in the middle of the night were indeed Arbery.
So yeah, they had more evidence than "you ain't from around here, boy" to go on.
Let me say again. McMichael had *VIDEO* of Arbery going through the house at night. He had several videos of this.