I'm a bit late to this discussion, but I still like to add that I think Sulla is amongst the most interesting of the Romans. I don't hate him like many do, but I don't like him either. It is my own personal opinion that he was one of the key figures responsible for the fall of the Republic.
Sulla, I believe, came to realise that towards the end of his life, that his descision to march on Rome had caused the Republic to become even more unstable. He had broken one of Rome's greatest taboos, to bring armed men within the walls; and that now there was no turning back.
He probably knew afterwards, that after he had taken that descision, that others would attempt the same thing. So, he renounced his powers and returned to public life, probably because he believed that he had done everything he needed to do. Maybe he also thought that if he wanted to be seen as the saviour of the Republic he needed to renounce his powers, just as Cinncinatus had done in the past.