So then why did he?
Again you can’t have it both ways.
You can’t claim that Kizaru will just follow orders to the best of his ability always and then claim next that he didn’t need to attack Sentomaru.
Saturn doesn’t order Kizaru “don’t attack Sentomaru.“
What actually happens is
Saturn: can you get in the dome through the light barrier
Kizaru: I can, but Sentomaru is below
Saturn: jump over him then
Kizaru: would rather not, I respect what he’s doing. Plus, Sentomaru’s in charge of the Pacifista and sea beasts
Saturn: hmmph
The two of them are planning the best way to attack the island and Kizaru actively points out that he should take out Sentomaru.
It shows exactly what I said, Kizaru is loyal to the Celestial Dragons, is following their orders and has no qualms about fighting. He saw a flaw in the attack plan that Saturn didn‘t and corrected it, despite the fact it matched him against his friend.
If he wanted to help Vegapunk, he could easily have said nothing and left Sentomaru in charge of the Pacifista.
If he wanted to save Bonney, he could easily have not kicked her into the laser field.