There are more guardians outside of the Grove. They're akin to... demigods, in a sense of the world. Not quite gods, not quite human... something greater. Moder and the River Guardian used to be corrupted, though on their deaths seemed to have been cleansed of this corruption. Though, going forward, killing the Guardians to purify them may prove detrimental. We simply got lucky with the first two. They were strong enough to revive. If we try again, we might not have this luck.