Well, you are right about the attack - I didn't expect that to happen. I expected Sasuke to hit the road, which would have been the logical choice. But in a certain strong sense this also made sense. Despite Sasuke now hating Itachi more than anything, he was still the big brother he alwaus looked up to in the past. And it was Itachi indeed who suddenly judged himself ready when he succeeded in a surprise attack. So, now Sasuke is ironically mirroring Itachi's behavior by surprise attacking those he was serving for three years, instead of simply leaving. And I wouldn't be surprised if his reason was ultimately the same: To see if he's strong enough.