..
I see you're one of "those" people, so I'll explain this for people with your level of intelligence:
"Shouldn't it be a Marine's duty to protect Celestial Dragons?!" - This implies that ANY Marine, not even just the Admirals, HAS to protect the CDs. They've got almost no say in this.
"It's only his accomplishments and stature that keep him from being
eliminated for this insubordination."
Gonna repeat that.
"It's only his accomplishments and stature that keep him
from being eliminated for this insubordination."
Sengoku, who's the Fleet Admiral, should know the pecking order by now, and he himself said that you can get killed for not helping the CDs as a Marine. The only reason Garp wasn't taken out is because he's too valuable of an asset.
And then there's this:
Kizaru admits that he NEEDS to capture the Straw Hats or the CDs would be pissed at him.
And when he doesn't succeed?
He has a fit and captures 500 other pirates to make up for it.
Safe to say, there's no reason to assume Kizaru doesn't protect the CDs and isn't actively proud to do so as well.
Just because we don't SEE him do so, doesn't mean he HASN'T done so.