I recently played Little Nightmares II and I LOVED it, but the ending utterly broke me. Dejected and looking for a place to vent my pain, I looked through the tags here on Tumblr, but to my surprise I found that people weren't only grieving -- they were angry. So many people are calling Six evil, a betrayer, a monster, and maybe there's some validity to that; she definitely has violent tendencies, but I think the ending is far more complicated than just a simple betrayal.
First and foremost, Six definitely liked Mono. I saw a few posts saying she never cared about him, but I don't think that's true at all. They bonded several times throughout the game and she trusted him implicitly, to the point that she reached for him when she was in danger and risked her life to save his. I read a few theories that suggest she betrayed Mono because when the Thin Man appeared, instead of trying to grab her hand to pull her to safety, he cowered and let her get taken. That could be true, but again, I think it goes deeper than that.
Incidentally, I also read a few arguments claiming she's evil because of how she acts, like breaking the bully with her bare hands, snapping the mannequin fingers, and warming herself by the pyre (which was morbidly hilarious), but I don't think these urges are her fault, much like the intense hunger isn't her fault. She didn't start having violent impulses until after the hunger consumed her, and we see that at the end of the game via her body language. She's ashamed.
So why did Six drop Mono? I have a few theories, but the strongest one seems the most obvious to me: Mono is the Thin Man. I read a few comments where people theorized he isn't the Thin Man who chases us, rather he becomes a replacement/successor. Personally, I find that hard to swallow, since the connection between Mono and the Thin Man is so discernable. For example, Six and the city folk couldn't connect to the TVs the same way Mono could. Mono was also the only one able to open the Thin Man's cell, he was the only one able to defeat him, and he figured out how to teleport through the TVs on his own. If that isn't enough evidence to convince you, there's also an achievement/trophy you can earn post-credits, where you literally put on the Thin Man's hat. Look at the description.
Then there's the subject of Mono's name. "Mono" can mean many things, but in English it generally connotes singularity, loneliness, one. When I first learned his name, I immediately thought it was wholly ironic, as he spends most of the game in Six's company. It also means (get ready for it) sound! To record or broadcast sound from a single direction. Now, this next part could just be me overreaching, but it's also used in words like "monochromatic", meaning a single wavelength of light or having only one colour. The Pale City and the TVs are entirely blue. Several shades of blue, but only blue.
Inside the Tower, the light turns ultraviolet, a colour caused by electromagnetic radiation that exists inside--you guessed it--old TVs! The colour ultraviolet has long since been associated with catastrophe, as the radiation is carcinogenic and even damages DNA if it comes into direct contact with skin. This could be why Six morphed into such a grotesque state, while Mono remained completely unharmed. Mono is the Thin Man. He has to be or he probably would've died in the Tower. This also explains why the Thin Man didn't kill Six outright, but instead locked her in a room full of toys. After all those years, he still considered Six a friend and he wanted her to be comfortable. In addition, the official character bio for the Thin Man lists his only alias as "Mono".
So why did Six drop Mono? Canonically (even if you change his hat), the first time Six sees Mono's face, really sees it, is during that moment on the collapsed bridge. She pauses, takes a good look, then lets him fall. I think she does so because she sees the similarities between Mono and the Thin Man. She realises what he's going to become, so to prevent that fate she lets him fall, presumably to his death. In a sense, she was trying to save him, and possibly the inhabitants of the city. The only question remaining is:
Did Six's actions cause a loop that inevitably lead to Mono becoming the Thin Man, or was it preordained and neither she nor Mono nor the player had any control over it? Maybe it didn't even start with Six. Maybe it did. We may never know, but I don't think her betrayal was vindictive.
