The main body of the god of killing had died to the church of justice over a thousand years ago. That was a long time ago, during which the god of justice has grown stronger.
But in that time, he hasn't grown any stronger. He is currently at most a half-god who needs sacrifices to keep existing, so there is no way that he can fight the god of justice now.
Now his lacking power will make him fail in his quest to resurrect again. He doesn't have the power to protect the vessel he wants.
It is not that he can't defeat the angel of justice sent to kill the angel of killing. He is sure that he can.
It is just that it is not worth it to expose himself for an incomplete angel of killing. If he appears and fights the angel of killing, he can be sure that the god of justice will send his godlings after him.
Those godlings will be able to kill him. Even if one can't kill him, the god of justice can still send more.
