Night's Mist was an ability that he couldn't abandon.
He needed it not only to stand against Chaos but also to thwart the plans of people like Rose, that endangered his life, and kill them.
So, although it was messing with his brain, amplifying his desires, especially the dark ones, and changing his character to something along the line of Chaotic-neutral, he wouldn't discard it even if he found a way to.
"My lord, we've eliminated the threats to your life... for now. As long as Rose isn't dealt with, there will be more like them who will come to kill you. We need to make a plan to get rid of her." Delilah was certain that the possibility of Rose acting against Roy once more was 100%, and that's why she came to the conclusion that Rose needed to die for Roy to live without worrying about assassins.
Delilah could be seen standing behind Roy, respectfully waiting for him to speak.
