What are you referring to?
Correct me if I'm wrong but it seems like dude had or got the knife after the gun was in his possession and after they left to get on the elevator. Didn't seem like the knife came into play until they dipped. I'll read it again SLOWLY.
Edit: I read it perfectly fine. They weren't thinking about a damn knife or being stabbed because he ran up on them while waiting at the elevator. Did I miss the part where he threatened them with a knife?