Sinestro War (1)

The Yellow Impurity and the ban on lethal force were measures the Guardians had put in place to prevent the Green Lantern Corps from spiraling out of control. Every Green Lantern Ring had a Yellow Impurity, becoming extremely vulnerable in the face of the power of fear and yellow substances. That was also why they had painted every building on Oa yellow. As for locking down lethal force, it was meant to keep Green Lanterns from having their minds consumed by killing.