I'm not the only one to say this (OP touched on it) but I think blaming Six's betrayal on her just being "sociopathic" or "sadistic" is such a surface level read. Reminds me of people who called Joel a "villain" after TLOU1, reducing richly developed and complex characterization into the 2-dimensional binaries the game spent 20 hours breaking down.
I'm not gonna try to do my take on the whole story or how what I'm saying ties into various existing theories. I'm just gonna talk about Six in a vacuum. Six acts pretty heartlessly at times in this game and you are meant to recognize that darkness and be unnerved by it. And yet, Six is a neglected and traumatized child. Underdeveloped socially and emotionally (practically feral at times) and trying to survive in a horrifying world out to kill her and the only other living human child she knows. By positioning the cruel, childish way she acts toward people you also view as enemies as signs of "evil" nature, you are ignoring that this girl has clearly lived a life of cruelty.
She acts far more like a child than the clear-headed Mono; she plays with toys, gets curious, defers to Mono's leadership, and — on the other end of the arrested development spectrum — attacks animalistically instead of using weapons and behaves vindictively toward your fallen foes. Is this kid broken? Hell yeah. But it's not her default. She wants to have a normal childhood, she wants someone to care for her and care for them in return, she wants safety and comfort.
So Six's betrayal in the end could be based on a number of things, depending on your theory — getting a good look at him without his mask/hat, having some monstrous change within her, whatever — but she did not make that choice just because she was salty about him hiding under the bed. He clearly spent the rest of the game trying to save her and got this far. Even if Six didn't have empathy (which I don't agree with), even psychopaths don't discard people they believe are useful. Maybe their trust would be fractured but she wouldn't suddenly be cool fighting alone.
I think they wanted to communicate three things about her character in order for the betrayal moment to land:
1. Six, whatever her reasons, has the capacity to behave cruelly. She has at least the capacity to let someone fall.
2. Six has the capacity to care for and trust in others. She has at least the capacity to risk her life catching a falling friend.
3. Six wrestles with a deep fear that can overwrite either of the above.
She recoils in fear after saving him from the TV the last time, perhaps sensing a difference in him with the release of the Tall Man. She was afraid of the world and found comfort through the music box in her little cell. Mono challenged that, essentially saying "we can be each other's comfort", and she took his hand. She started to fear Mono and, after being captured, retreated to the comfort of her fantasy: the music box and a sealed room. When Mono arrives in her fantasy room, his presence threatening to once again tear her from safety and comfort, she recoils much like she did in their first encounter. But when Mono calls to her the first time in this state, she presents the music box to him. She wants the safety of her locked room, the comfort of her music box, and the companionship of her only friend. Most of all, she wants those things for him too.
THIS LITTLE GIRL IS NOT "EVIL".
And even if her choice at the end was an act of intentional malice (depending on your theory), it is not the result of an inherently evil nature.
imo lol