Satan actually plays a pretty tiny role in the grand scheme of things. It's not quite accurate to describe him as "the greatest evil." Satan really doesn't show up all that often in the Bible's writings. He is mostly portrayed as a deceiver and very little more. The more I read the Bible, the less I'm impressed with Satan's role.

The greatest evil, by contrast, is human sin and the death that resulted from it.

Why would god create satan?

If God is an omnipotent being than he should have known about satan's plot in introducing sin to humanity. If this is such a bad thing, why did God create satan knowing that he would become the greatest evil. Or even just intervene when Satan was convincing them to eat the apple.