Lt. Aldo Raine: You probably heard we ain't in the prisoner-takin' business; we in the killin' Nazi business. Business is a-boomin'.
Lt. Aldo Raine: The German will be sickened by us, the German will talk about us, and the German will fear us.
Lt. Aldo Raine: Each and every man under my command owes me one hundred Nazi scalps... And I want my scalps!
Answer: Based on his willingness to join the Basterds, we can assume he killed them because he didn't believe in what the Nazis were doing.
There is a cut of him being whipped during the bar scene. I think he was taking revenge for harsh and perhaps undeserved punishment.
No, the whipping is how it feels in his mind sitting next to that SS officer and having to pretend to like him and be a Nazi. He's ready to snap.
lionhead