Kill the little devil!

My daughter is bright and inquisitive. She wants to know how everything works and how things are made and why is this like this. I feel like I spend my days fielding questions from out of the far left portion of her brain.

How are clocks made? (I don't know)

Why does water look blue in the lake but not in my cup? (it has to do with the reflection of the sky and the length of light waves)

How did the baby get in there? (God put it there) How will it get out? (The doctors will take it out)

Do you think cats like cat food? Or would they rather eat pizza? (i have no clue and we aren't going to offer them the pizza)

But occasionally she asks things that really make me start to think. . .

Mommy, how did the devil become the devil? If he was a bad angel, and God kicked him out of heaven, why didn't God just kill him?

I gave her the pat answer. . . God wanted us to "want" to do his will, not do good because we didn't have a choice. And Satan provides that choice.

She accepted it - but it really made me start to think. Why did God allow him to go free? Why not kill him and keep us from a life of pain, suffering and consequences? Did he really want our freely given love so much that he was willing to loose so many in exchange?