He does not have to choose them to be evil. The are so of their own accord. He need only choose which to save.
He KNOWS before we are born what we WILL do, and prepares us so that what we do fits into his great plan.
If God is merely reactionary, then why does Scripture portray Him as taking an ACTIVE role in bringing people to salvation?
He doesn't have to choose anyone to be "evil", they all naturally are! That's what Arminians choke on all the time. They say Calvinism paints God as a cruel and arbitrary despot, choosing some to save, and actively damning the rest. The fact of the matter is, If God didn't do a thing, didn't chose anyone for salvation, they would all be rightly on their way to Hell, BY THEIR OWN CHOICE! God doesn't have to decree anyone to Hell, they're already naturally on their way there. There is no neutral ground. Man is born a sinner, and sins at his earliest opportunity, and continually. Man's will is not able to choose against it's very nature. The natural man is at enmity with God.
There is none righteous, no not one.