That's what I actually love about the game. It takes corruption to next step and shows the vulnerability from both sides (Li and MC) and shows us that they aren't just bags of flesh but rather multifaceted individuals. Not sure if you saw the scene with Mars hugging Vi, that perfectly encapsulated their struggles. I feel bad for Erin too ngl
Seeing Mars cupping his face in his hands in depressed exhaustion during the encounter where Cecille uses hypnosis for the first time really affected my opinion of him. He never says "I am going to ruin his life and enjoy it." He expresses how unhappy it makes him. He tries to find every way conceivable to avoid the inevitable before following through with an action. He works really hard to get there, too.
Mars is a caring and unfortunate individual who happens to have a traumatic backstory of abuse, serious parental issues, and an inability to harm others without requiring a justification - even one he's making up just to make it hurt less.
This is the only game with this tag which has ever gotten a free pass from me. In any given character route, Mars works himself ragged to get them to like him by being supportive and present, and then he makes promises to care for and to watch over them. He's been told his whole life that he's a demon and therefore irredeemable, but he spends his whole life trying to find comfort in people around him and build them up into a family for himself. His reasoning is always predicated on what he thinks another person needs, and what would be best for them - even going so far as to tell them outright that what they're doing is wrong and not something he wants to support.
The game actively punishes you for being a soulless monster. Some people you encounter in the game already have that covered for you, unfortunately, and the only bad endings are ones you make by being inconsiderate or cruel. Anyway, my favorite part of the game is finally getting to where we (as Mars) finally get some of that kindness and support reciprocated by other characters. We all know Mars needs that after being put through the wringer.
In a different world under different circumstances, Mars would have been a great friend to Erin. For being a demon, he sure spends a lot of time rescuing people, forcefully or not. Isn't it weird how he fills that role so well?