So many people are misunderstanding Kizaru’s conflict.
Kizaru wants the SHs to escape with Vegapunk, but he’s not just going to throw away his duty like kuzan did. He showed from the start that he was willing to kill Vegapunk.
That is why Luffy vs Kizaru has to happen. If Kizaru betrays Saturn, he will be committing treason and he will probably get hunted by the remaining admirals/gorosei/holy knights. But if kizaru is defeated by Luffy, then vegapunk gets to escape and Kizaru doesn’t get branded a traitor.
It’s literally WCI 2.0. Saturn is the main villain, but Kizaru is Luffy’s 1v1. Saturn, like BM, has pretty mid offense(by top tier standards), but is insanely tough to take out.