Same reason why zoro is there.
Killer >> kamazou killer
Killer >> kamazou killer
They couldn't break Killer so they had to use his captain ad hostage to force him to work for them. Killer was fighting to protect his captain which means he would have been stronger than when he originally faced Kaido. One piece characters grow stronger to protect their friends.
Zoro is working to be the WSS. He's already beat or bested 3 supernovas. The only thing Law and Kid has over Zorol is they are captains. But that's irrelevant since Zorro isn't working towards being a captain. Neither one have shown anything that places them above Zoro in strength and power.