It's definitely a feeling of "shouldn't but totally will" that pushes Mettaton to look into what he figures is Mikan's room when he passes by. He's seen a small sliver of the intense undertows that flow under Mikan's ocean and he must admit he is... curious. What reasoning does she have to hate herself so much?

He opens the door, peers in, and his eyes widen as he watches Mikan spring her different traps. It's chilling. Not only to see her usually tentative and cautious face twisted into such sick derangement, but also to see people he knows, people he recognizes from Wonderland, fight her off. Though there is one human male who gets some trouble that makes Mettaton smirk a bit. Good. Take that, asshole.

He's not entirely gobsmacked, at the very least. Her behavior reminds him of how she was back in Quor'toth, though that's not a memory he likes thinking about much. After watching the memory he closes the door, contemplative.