Sirrus and Achenar are both mass murderers. However, Sirrus qualifies and Achenar doesn't because, in Myst IV: Revelations, Achenar finally feels remorse for his actions and does a Heel Face Turn and Heroic Sacrifice, whereas Sirrus not only seems to have no regrets but commits additional atrocities: kidnapping his own sister, taunting her about how he plans to suck her soul out of her body before doing a Grand Theft Me, and then holding her soul captive if the player manages to get his soul out of her body. He taunts her about how he plans on murdering both her and their parents, and in the bad endings where the player loses, he murders his brother and the player.
Gehn from Riven is not much better. He's an evil Dimension Lord who feeds his subjects to a sea monster. There can be little doubt where Sirrus got his evil from...
Esher from End of Ages. Imagine if Doctor Mengele had found the secret to incredible power...
P'aarli in Myst: The Book of D'ni. The Terahnee may qualify as well, although most of them are still somewhat uneasy, even if it's only on a subconscious level, about what a living hell the slaves' lives are. It is the P'aarli who do the actual kidnapping, dehumanization, and murder. Any wonder Ymur crossed the Moral Event Horizon within days of Terahnee's fall?
A'Gaeris. He convinces each of two already-estranged friends that the other is a criminal, all so that he can have someone bitter to help him destroy D'ni.
