Why was the man shot?

The last half of this film (the science fiction part) is quite straightforward, but the first part is confusing to me. What was the point of shooting the man who comes to open the door for the brother?

Now that I think about it, I do remember him saying to Bruhl that he wanted to be paid. I suppose this could've been a way to get rid of him, and the brother just happened to show up at the door and provided a convenient person to blame for the crime. But of course that depends on him picking up the gun, which I would think a sensible person would not do in that situation.

"Extremism in the pursuit of moderation is no vice."