That is the exact same interpretation I always had of that scene as well DC. Sonny was on quite a power trip in his family position before Vito's assassination attempt, controlling the streets, taking down rival rackets, all the other activities a blood thirsty young Mafia man would have reveled in. Once Vito got shot, the days of care-free aggression were done for him, and I think this is the realization he comes to as he's looking at Vito's empty chair.