Do we even know for sure that Gray Lord's decree is about killing in general? What if it just affects people who try to kill demons?
Like Gallon said, Fraudrin definitely went for the kill when he attacked first Deldrey and then the group as a whole, there's no ifs here. He clearly intended to kill in that moment even though he told Denzel minutes before that the Pleiades can't kill him, Fraudrin, with Gray Lord around.
I think it's possible that Gray Lord only affects those who try to kill demons, it makes a whole ton more sense as an additional means of protection and it wouldn't prevent the demons from killing their enemies. Keep in mind that GL fought in the war, it'd be incredibly dumb to prevent your elites from fighting without holding back because they don't want to accidentally kill anyone. Not every commandment needs to he as ass backwards as Galan's, there's no room whatsoever to get around his decree after all, I doubt ALL of the curses are like that because they'd hold the group back way too much whenever they're all together.
This could also explain why the TC, despite beating Meliodas down pretty badly, don't finish him off for good and why they're so calm. They have him completely tied down, he can't flee and he can't retaliate with his stronger attacks if he doesn't want to get shit on by Gray Lord.