Warlocks are typically mages that have relied more and more on fire magic until they gave in to more powerful demonic magic. Kael'thas is a good example.
I see warlocks on both factions as figures who fight for their side but secretly only for their own personal gains. So I don't see them as the good guys in anyway, don't quite know of Blizzards standing on this but they always like to reverse and change things around so who knows.