Slater was in a grave mood, with pursed lips and a thoughtful expression on his face as he brushed his mustache with one idle finger. He stared ahead thinking of Vivian and Harry - if Harry had been as heartless as Viv had said, and Slater had no reason to doubt the woman's words, Harry needed to be punished. Hurting Slater's girl resulted in Harry having one of his girls' hurt - he wouldn't hurt Mary, not at all. So that left Ann, and as Jimmy was the closest source to the woman he'd use that to his advantage.
On hearing Jimmy knock he rastled "Enter" for his son to come through.