I think the problem is remarks such as if God is all powerful, why did he create Satan? In my opinion, it misses the point of the war in heaven.
Say you were married and had a family. A lovely wife or husband and two children, ages 2 and 3. Now these kids are adorable, but their reasoning ability is a little limited due to their tender years. The 2-year old decides to "take over" the family and try to make decisions for you and your wife/husband. Now does someone say to you, the head of the household or co-head, "If you know what you're doing, why would you create such a little monster?" No, because it would be inappropriate. The kid is perfectly fine being a kid, but not as being the head of the household. Thus to be the true leader, in that you know what's best for all, you put the kid in his place - that is, being a kid. So, too, with this "Satan" creature. He tried to "take over" heaven and was thrown out and locked away. And when Jesus had dealings with him, he told him to "get behind him," in other words, to follow not lead.
Logically, we can deduce at least three things:
1. The Satan was created for a reason.
2. That when this Satan stays in his place he's not a problem.
3. That he has a purpose, but the purpose is not to lead but to follow as in not using this part of self to make decisions. They're always going to be disastrous to self and others.