The mirth is gone from his face now, a dire seriousness dressing his features. The machine continues its deadly work and Mettaton looks Sans directly in the eyes.

"You know how, Sans. But you refuse."

He adjusts the machine, changing the levels and blatantly taking it out of the 'safe zone'. He hums a bit as he does, checking other aspects of the machine before looking back to Sans with that same severe look from before. He opens his mouth but the voice is... different.

"Six seconds, Sans, we're almost there."

The beam increases, growing brighter and brighter, and the last thing Sans sees are Mettaton's metal eyes glinting in the light as he mouths Three...two...one!