I Am Your Worst Nightmare
After seeing that the number of Saints in the Sacred Land wasn't that many, the Champions of the Light decided to carry out their mission as soon as possible.
One of their members had extraordinary hearing, and he managed to pick up what Dracul had said earlier.
Since the Guardian of the Ancestral Ground had no intention of protecting their target, they no longer had any reason to hesitate and began their operation.
Of course, they also heard that Kieran might help Gaap, but that didn't matter. There were six of them, and dealing with two threats wasn't that big of a deal.
The High King of Lunaria even ordered for Kieran to just be contained and not killed. Even though they were not afraid to have an all-out war against Memento Mori, they would still consider this as a last resort.
Gaap wasn't a member of Memento Mori, so technically, they weren't breaking their agreement.