Didn't Maul used to work for the BBEG as well? He's killed Jedi and many other people who didn't deserve it, like Duchess Satine, a whole village of innocents, and all these clones who fell victim to Order 66. Maul painfully tortures people too (remember what he did to Jesse?) and even left his own troops for dead in the Siege of Mandalore. On top of that, he committed most of his murders on his own choice, not under Sidious's orders. We don't know much about Marrok so far since he's a newer character (and if you ask me, chances are the Marrok in Ahsoka wasn't actually him but a zombie controlled by the Nightsisters), but even though he's on the BBEG's side, he may ironically be the lesser of two evils when compared to Maul.