I would think that most of the "evil men" who joined Sauron had joined him before, considering they may have been disorganized. Sauron seemed to be a god-like thing and be on the winning side, which they wanted.
Plausible reasons for being:
Easterlings: Rumor has it that throughout Rhun are temples to Morgoth and his kin. I suspect that the Easterlings were merely following through on their worshipping of Sauron.
Variags of Khand: Barbaric, possiply leaderless, wanted to be on the winning side, dared not oppose Mordor for fear of losing.
Dunlendings: Same as above.
Haradrim and Corsairs: The only people who we know had a grudge against Gondor, their reasons are obvious, included in some of the reasons above.
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"What mortal feels not awe/Nor trembles at our name,
Hearing our fate-appointed power sublime/Fixed by the eternal law.
For old our office, and our fame,"
-Aeschylus, Song of the Furies
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