Hey. When you're caught, you're caught. Mettaton resigns himself to his fate; no wonder Toriel had been queen. She still had that air about her that demanded respect.

"If I tell you, can you at least assure me that you won't be too unkind to her? Alphys has made mistakes, but she isn't a terrible person. And she is harder on herself than anyone else could be."

He's mostly concerned that if Toriel dealt with the former royal scientist's deceit too harshly, then Alphys would take a severe blow to her self-esteem. Or what there was of it, anyway.