Someone I know posed an interesting question, which I had no answer for. She's Catholic, but was just curious after watching a few episodes of "Haunting" (a tv show on Discovery channel, which explores true stories of demonic encounters and exorcisms performed, some by Catholic priests). The question:

"Why did God put Satan and his demons, and us, on Earth together in the same place?"

Thanks in advance.

The demons have no bodies. I don't think they can be said to be "on earth."

Maybe she means why does God allow the demons to possess us?

That, and/or why does God allow evil to be near us?

A partial answer:
* God lets Satan attack Job and then uses the attack to teach Job and his friends a lesson.
* St. Paul talks about handing somebody over to Satan (excommunicated?) in order to learn not to blaspheme.
* There is a mutual relationship with people and Satan. Christ told some that they were of their father, the devil, and they wanted to do his desires.

Hope that helps.

Evil isn't a thing, but a mere absence of goodness.