Ask them about their characters. Find out what their motivations are. Evil doesn't mean psychopath.
I have played in an evil party (highest alignment was LN) we managed to work in the campaign because for the most part our party were selfish bastards out for their own interests. We would do anything to get the job done: torture, bribes, lie, cheat and steal. Nothing was below the belt.
The DM switched the motivation from "save the world because you are good!" to "We'll pay you a lot of money if you stop those guys ransacking the city."
We pretty much became black ops for the government doing the jobs that no one else wanted or were willing to do.
Our overall journey may have saved the world but that was only because the world happened to be the place we lived and by the way that is a million gold saving the world fee either in a lump sum or with a 3.5% interest compounded monthly until payed off I'll have my people contact your people.