You Saw Sans Infront of the door, holding a knife?, he killed toriel, why??

Sans: looking at the door ...

Sans: turns around and about to take a step and sees you Oh.. the human... didn't expected you here...

Sans: don't mind this...

A pile of dust in on the ground

Sans: I killed Her...

Sans: laughs a bit crazy quietly your next...

Sans is prepared

What do you do?