That is...definitely an unsettling smile. Not enough to deter Mettaton from what he intended to do though. He wasn't intimidated by Chara.

He wasn't.

So when they tear another flower away from their body, he lets out an annoyed sound, moving forward and extending an arm so as to grab Chara's.

"You need to stop it, or I will make you!" he snaps.

The fact is that he's extremely attached to Asriel, and knowing how upset the young boss monster would be at seeing Chara again, this time self-brutalized and covered in blood and flower petals...it's what motivates him right now.

Asriel's been hurt enough. Mettaton won't allow this without at least trying to stop the outcome! What sort of friend would he be if he stood by?