No, I wouldn't say a kill is the only way to confirm him. But he hasn't done much in his threadplay to have me read him as town, and he has some degree of evidence against him too. In general, I'm pretty against the notion that you just confirm people through kills, it's a losing strategy.
If hes arthas, you can assume just about any role on him will fail and well have to kill him to be sure
But yes in general this is correct (this is for arps benefit, not yours lol)