The crafty Bill had been quietly observing the two all along, not intervening even when their conversation took on a slightly explosive tone. It wasn't until he was sure David took a step back that he moved forward with a hint of regret and deftly changed the subject.
He actually had been looking forward to Tommy enraging David, and then to David teaching Tommy a lesson in Miami, because then the old man would owe him another favor.
Unfortunately, the sly old dog saw through his and Diane's little schemes. Indeed, he was irritated by Tommy's response, but once he realized that neither he nor Diane had stepped forward immediately, he keenly sensed that Tommy's latter words could very well be a trap they had set, meant to provoke him into making the situation worse, and then, of course, he would owe the couple yet another big favor.