He would never have dared to attack her at that hour, if he had not found her alone.

His mind was made up.

He would be more at ease alone with Mademoiselle Stangerson in the pavilion, than he would have been in the middle of the night, with Daddy Jacques sleeping in the attic.

So he shut the vestibule window.

That explains why neither Monsieur Stangerson, nor the keeper, who were at some distance from the pavilion, had heard the revolver shot.
"Then he went back to The Yellow Room.