Great age old question. Why didn't God send Satan elsewhere? The God of the Bible made humans and angels with freedom to love and obey Him. As you said, some of the angels rejected God.
Scripture says this not I. Does it not seem strange though that to date, all the conscious beings said to have been created by God have rebelled? Could it be that God made tem/us that way on purpose.
Remember that without eating of the tree of knowledge we cannot get closer to knowing God and that withot knowledge of good and evil we would lose our moral sense. the same moral sense that all churches and religions try to teach.
Not all people reject God. They just reject remainning ignorant of good and evil. Thank God for that.
If the serpent hadn't tempted Eve would they ever have sinned? We can't say for sure. But according to the story of the Bible people chose to reject their Creator and that's why there's an obvious big-time problem with humanity.
What problem. God told us to reproduce and learn of good and evil. We do both quite well. If God were not pleased, He would let us know. Scripture says that all of God's works are perfect. Will you stand before God and tell Him He goofed.
Maybe the reason you assume God must be a genocidal maniac is because you assume people are basically good and underserving of punishment. The story of the Bible disagrees with that assessment and never blames God for the evil that exists.
It says that God is the creator of all things. That would include evil. The God of scripture is resposible for all of the perfect works around us, including evil. He created the tree right?
Someone has taught you that angels had sex with women. That's based on a very shakey interpretation of Genesis 6. The "sons of God" are more likely the sons of Seth of chapter 5.