Hasn't it occurred to him that maybe Barnabas would like to be consulted before being given a massive injection of heaven knows what? And golly, he's sure of himself.
Doctors almost always want to bulldoze their way through situations, and back then, fewer people were questioning this. People weren't talking "patients' rights" yet. Mainly, though, I think Lang didn't want to try to have a conversation with an out-and-out vampire. As far as he was concerned, he had a Dracula in his hospital, who was about to start wreaking havoc. I doubt anyone would have thought that a patient has a right to continue to be a vampire, if anything can be done about it, even if it's a risky treatment. Once he's "human", then he has rights, I suppose.
As for his self-assurance, Lang is pretty typical in his attitude (amongst doctors), even down to his strange (booming) vocal style. Doctors live in their own strange world separated off from the everyday world, where they rule, and when no one questions what you do, you get weird, because you care only about your own judgment, not that of the outside world... maybe you even start to talk funny.
We're up to Lang already? If only we'd gotten much more of those kinds of moments with Barnabas at the window, where he came to appreciate one thing after another about human everyday life, and we got to see it all change him.
