Well I don't think it's that evil. The mothers make honest mistakes. They can't tell which babies are their own, so they assume all babies in the area are their's. So when they see another rat with "their" babies, they think the other rat will hurt them, so they fight to defend their babies.
Now other rat, or rat mama number 2, thinks that rat #1 is there to hurt "her" babies, so she defends them. Each thinks the other will hurt the babies, and each is a mother, so both are willing to die "saving" their babies. It's not evil, it;s a sad truth of motherly love.
Lila
