Ah.. I forgot what I was going to say this morning.
Something like-
Some Related Spoilers
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Say the Witch is the ruler of Narnia-
Wouldn't it be just as "evil" to not turn Lucy in?
As cruel as his punishment, he already knew what would happnen if he didn't.
So, in the beginning he already knew he must turn Lucy,
or if he was really good, he would just send her back in the first place.
What I mean is-
If Tumnus was really "good", he would never have brought Lucy to his house.
Plus! He brought her again.
Pushing the limits and risking.