Mettaton is completely frozen as Mikan speaks. He can't deny anything she's saying. She's right. He knew, the whole time, from the moment he passed by her room, to the second he turned on his heel and walked to it, to the impulse that pushed him to open the door, what that meant.

"I knew the general idea of what would be in there. Whatever your worst memory would be. But I didn't know the specifics. And so I opened the door."

She looks so upset and Mettaton looks away. He can't bare it.

"It was just as easy to open the door as it would have been to walk away."

Speaking the truth doesn't always make things easier, apparently.

"I was curious about what reasons you'd have to really hate yourself. Why you think of yourself with such disgust. After seeing that memory, I'm not surprised you'd think so low of yourself if you're capable of that savagery. Though honestly, while it was frightening to see, I can't find myself in any way able to criticize- I was completely and willing to murder Frisk because I was sure that with their soul in my possession, I'd be able to defeat Asgore and leave all the other monsters alone in the Underground to become a star on the Surface."

He blanches.

"I don't know what was compelling you to act as you did, but I can't judge. I shouldn't have broken that promise."