I was torn too! But I have an mc for an each RO and they all made different choices so I got to see all the different endings. It’s really nice when no choice is just obviously the “best” one because it lets you roleplay and make the choice your character truly would, consequences and all.
Guide below for anyone struggling with the Mark stuff 
luckily my Mark-mancer happens to be super paranoid and unhinged so I had a good first run but I imagine only specific MCs will ever trigger this path (which makes complete sense considering what it entails…). In my testing (thank you to cogdemos for the back button) I realized you do have to make pretty specific choices.
These choices take place once you hit the lake house.
- When MC is questioning whether Mark owns a lake house, you can choose “what else don’t you know about him” or “this development gives you the creeps”.
- Ask if he planned the corpse first or the fishing trip.
- Call him batshit crazy or creepy.
- When thinking about Mark, you can have mc either be lured in by his darkness, think he’s wicked and love it, or that he gives them the creeps.
- Do the threesome with Becka and call him a psychopath.
- After the threesome, ask if it changed anything between you, then tell him “I know who you are.”
- Finally, for the “unmasked” version of the ending, let him kill Becka and look away or stop him by touching his wrist.