Albus has deceived him from the start, in order to confront him with his own sick thoughts as punishment. He pretended to give him orders and control him before their shag, only to tease him with his tight arse until Scorpius betrayed himself. He suspected it all had a purpose other than just testing the runes, and in hindsight he is sure of it. The strangeness Albus had exhaled like a foul odour from the start, wasn't that a clue?