But you can't rent out a murder room, with the explicit understanding that this is his murder room, to be used for murdering people.
I think, like another poster said, that it might depend on if the lease spells out the purpose of the business. If the nature of the business is known and illegal then Federal law isn't going to enforce a contract for that. I don't know enough about the law to know if this case is heard under Illinois law, but I do know that Federal courts sometimes evaluate cases under State law, so you might be right about that. I guess it depends on the issues raised in the suit. I haven't actually, you know, read anything about the case.