I agree that Ryoguyu's "Lethal Justice" means he intended to kill everyone in the prisoner mine. However, just like his fight with Momo and the Scabbards was interrupted by shanks, his fight against King and Queen was also interrupted. Although they were already defeated, he still intended to kill them, but got distracted in the middle of doing so by a phone call setting his mind (and ours) on the next target already, without looking back to double check that his roots really finished the job. And they could have indeed finished the job even without him being present, if King had an ordinary human body.
But King is a Lunarian. His body can withstand any natural environment, and easily an environment of trees and roots. He wasn't even seen shrinking, let alone dead. When Ryoguyu left, he could easily release himself from the tangle of root, and since his relationship with Queen is very similar to that of Zoro and Sanji, my guess is he's release Queen as well.
If Ryoguyu came back to take them (alive or dead) to the ship ordered to wait for him, he might very well not have found them there.