I dont know. In my opinion, hes been under pressure this entire time to kill VP.
But he's not only failed to do it, but also got the pacifista orders overwritten. This forced Saturn to make a move. Im saying that he is reacting to him failing his mission on VP already.
So my question is, how does Kizaru and Saturn react? Is it because Saturn is scarier than Kizaru? The implication seems to be the case since they all stopped fighting when Saturn gave the order.