Purity...? What do you mean by that?

[Hello I am a mouse and I have no concept of 'pure' or 'evil' beyond 'that is a predator and they are going to eat me' or... well, politics. But that's only because of the rats!]

Everyone takes care of themselves or their family. That's just how it is in the field. If someone else's children get eaten by a cat, that's unfortunate, but it's not your responsibility. You can't take care of everyone... if you do, you'll hurt your own family.