Michael was always two and three steps ahead of everyone else in his thinking. I'm sure he'd already considered the prospect of Freddie having been humiliated by being passed over by his "kid brother," and having been put in his place, back in GFI, by his younger brother in front of Moe Green. I'm sure Michael figured out that Freddie was a weak person, not exceptionally bright, and certainly prime fodder to be coaxed into betraying the family -- or at least compromising it.