Author set it all up. Kamui could have ended this at any moment but wouldn't leave Artemisia's side just in case Ragna popped out to kill her. I completely forgot that if Ragna emits full power for even a moment then Kamui would learn his location and head right to him, and it seems he forgot that too. The only "choice" the author made here was in forcing Ragna to use full power to hold back the explosion.
I guess Kamui could have insta killed Ragna but decided to use these next 9 seconds to dick around a bit.