Why does Stanley's attitude in Scene 8 transition from joy to menace?

Stanley's attitude at the start of the act is joyful. He has all of the information he needs about Blanche and has every intention of using it. His joys quickly transitions to menace when Stella refuses to believe him. He illustrates this menace when he purposely tells Blanche her present is a bus ticket home..... the exact thing Stella did not want to hear. All Stella wanted was a quiet evening of celebration. Instead, she got a truth she didn't want to hear and saw what she considered unnecessary cruelty from her husband toward her sister.

Jill D you're my savior thank you