Because he didn't want too. Kizaru was able to kill VP several times. When he used the clones to distract Luffy he was in front of VP literally talking to him instead of slicing him up.
Edit: my bad. It was after Kizaru kicked Luffy away.
He did kick him out of the barrier, and that alone took a lot of Luffy’s stamina to deal with. It seems like in that situation they could definitely have moved VP out of the way at least somewhere far enough for them to outpace Luffy running around the island.
Kizaru definitely didn’t want to be the one to pull the trigger, he wanted to get information, destroy what he felt like he needed to, and hand him over to be judged by Saturn rather than kill him himself.
To the OP, this all depends on whether the iron giant is there because Luffy can use him as a Summon