The term "Hero" is arbitrary, especially in this case.
The beauty of Roleplaying games is it's ability to morph around the goals of the players, rather than the players setting their mind on the goal that the game has offered them.
If your players want to be evil, let them, encourage them, and offer rewards and campaign options for doing evil the same way a good hero would be for doing good.
Completely and utterly reverse the roll of Good guys and bad guys. Maybe send the characters on quests of personal gain, or to serve a tyranical overlord who rewards them for their efforts.
While good players gain a steady supply of allies, evil characters would inversely gain a steady supply of enemies. Maybe a party of adventurers is sent to stop the players.
When offering allies to the players, you shouldn't be thinking about what the ally would get out of the alliance, only good guys think about others. You should be offering allies which would result in shear personal gain for the players.
When dealing with evil adventurers; "Because it's the right thing to do" is irrelevant poetic nonsense. On the other hand; "Because it can lead to wealth, fame and above all POWER" is thinking in the right head-space.