I do say it's not a threat: the sheriff has no intention of harming the guy, and if someone kills him, the sheriff will investigate. He's merely venting his own and the community's outrage, in a statement which is harmless because it's self-evidently true--if someone wants to rub the guy out, a town full of cops won't make any difference.
I would agree that the sheriff intended to sound macho. I don't know if his job is elected--some sheriffs are--but his remarks probably resonated with the locals. Nevertheless, the guy has nothing whatever to fear from that sheriff.
He could, and maybe should, get in trouble for saying things that suggest he might be less than diligent investigating the guy's death, but that's not the same thing as an actual threat.
Do you think he's encouraging vigilantism by coming out and telling everyone that he's not capable of protecting this person? Sorry, it still sounds like a veiled threat to me - "if you move into this town, don't expect that the sheriff's department will protect you." It wouldn't be so bad if someone else had made the statement - but the sheriff is in charge of protecting the people in his jurisdiction. When the person in charge of law enforcement of a town comes out and says he can't protect someone, he's basically telling other people to have at him since the police are incapable of protecting him.
Would you have any problem with the sheriff making the statement about other people (like the black man quote I referenced above?)