What do I say without frightening my son?

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Today my youngest son (who is sometimes curious beyond his years when it comes to spiritual matters) asked me "why does God take some of the nicest people to heaven and He lets some of the most evil walk this earth?" His question was spurred I suspect by local news of a woman who killed one of her sons and tried to kill the other while lying about "being attacked". He also has on his mind the fact that a friend of the family just passed away of cancer. How do I answer questions such as this when I honestly have asked similar questions myself of late? Any ideas?