Not surprising. They were clearly the aggressors in this case. From what I understand, they confronted Arbery because he matched the description of someone who was stealing from a nearby construction site. That in itself is ok ish, had it been nothing more than a casual conversation, but they did it with guns drawn and weren't planning on letting him leave if he didn't want to talk. A scuffle happened and Arbery was shot. They claimed they were attempting a citizen arrest, but that would only ever apply if you actually witness the person committing the crime, which they did not.
Just FYI, they were convicted of malice murder and felony murder. Georgia does not have degrees of murder. The prosecutor doesn't have to prove intent to kill in either case. He only has to show the defendant had ill will, which of course can involve but not necessarily require intention to kill. Felony murder in Georgia is typically applied when someone gets killed while committing another crime.