You can find all sorts of quirks with Vader in a ANH in contrast with ESB. He seems much more emotional in the former, and while ruthless doesn't seem like he would kill his own officers at the drop of hat. (In fact given that Vader only kills officers via force choke in ESB, you could easily imagine that he's just really desperate to find Luke and typically wasn't like that through out his entire run in the Empire).

Thinking about it, Darth in ESB and on is really a combination of the physically intimidating Vader and the cold, ruthless Tarkin from the first film.