This isn't difficult. Firstly it wasn't "they" who killed him, it was Travis McMichael who killed him, and he killed him because Arbery assaulted him and attempted to seize the shotgun away from him.
They were interested in Arbery because they thought he went into that house to burglarize it. I don't know if they were aware if he had a hammer or not, but they probably saw it when he dropped it on the road as he was running towards them.
Quite frankly the two of them are dumb.
It is dumb to concern yourself about your neighbor's property being stolen. It is dumb to take it upon yourself to try and stop a theft of another man's property. It is especially dumb to confront a thief if he is black and you are white because it has a great risk of turning into an incident.
It is much safer and smarter to hide under your blanket and let some savior rescue you from all the bad people out there, but Gregory McMichaels had been a law enforcement officer for 35 years, and for him it probably felt normal and proper to pursue a criminal suspect.
"Dumb", "Brave", sometimes they are the same thing.
Thanks for the info. He was not stealing from a neighbor but a newly built home. Possibly not even locked. Just saying. Needed to get those facts out.