I believe that Shadow Six was a representation of her power, what was left of The Transmission's effects on her. She has no other reason to do this as Mono had done nothing but helped her throughout this adventure. She may have understood that if Mono left with her, the hunger may become too much for her, and so she dropped him to save him from herself. When Mono destroys The Transmission, her powers are no longer suppressed, but the murderous hunger that comes with them is also reawakened, as shown in the final scene. Many people would want to know, why did she drop him? There was an official statement released back in 2019 that stated that Mono was helping Six because The Tower was suppressing her powers and causing her to 'Waste Away', leaving her without the abilities she gained in The Maw. The final scene shows Six escaping into someones living room, where a dark counterpart, dubbed Shadow Six, gestures towards a poster advertising The Maw, and Six's stomach starts rumbling again. However, when it looks like they're going to escape, Six seemingly betrays Mono, dropping him into the tower and leaving herself. After saving Six from The Signal Tower, she and Mono try to escape the tower as it literally come to life and attacks the two.

If you are still reading you've either seen the ending or wish to know in advance.

It is intended to help those who do not understand the implications behind the ending of the game. If you wish to find out how the game ends, do not read this post. This post contains spoilers for the ending of Little Nightmares 2.

Little nightmares 2 ending