It is said that the powerful defender of the forest is a tree and cannot move. Legion isn't sure about this, but even if it is true, they suspect that the angel of killing should be able to move and wipe out this city in a few seconds if they liberate their divine form.
If what they suspect about the power of the divine form is true, then either the angel of killing can't unleash their divine form, or they don't think it is worth it to unleash their divine form just to wipe out the city.
Either way, these events have taught Legion that as long as they push the legends to the limit and they don't make them desperate, they won't liberate their divine forms. So they don't plan to try and kill any legend anymore.
For now, they are content with meeting with Sir Kris and having a heart-to-heart with him. If they can't even achieve this in his current state, then they have no business trying to take on the city.