To me though Beckman's badassery pretty much goes away if Kizaru was just putting up his hands in mockery, knowing that he wasn't under any real threat from the gun pointed at his head. It makes Beckman look like a fool more so than a badass if he says "Don't move a muscle, Kizaru" and Kizaru just complies because he feels like it rather than because Beckman's strong as hell and a real threat. While on the other end it does up Kizaru's level of badassery, I feel that Oda was meaning to show us Beckman's here.
If Kizaru thought there was no risk in continuing to attack Luffy, I don't know why he ever stopped to begin with. Remember, Kizaru came for Luffy of his own accord several times in the war, he wanted to take him out for the Tenryuubito incident. Even if he can perhaps be a bit random at times, I'd like to think that he stopped because he knew that if Beckman fires that gun, he gets hurt.