I'm suggesting that IF a dictator agrees to step down and turn his country over for free elections, and IF he is not a complete beast, and IF the people who take over agree to give him amnesty, then they should keep their word.
If you tell a dictator he will have complete amnesty if he steps down, and then break your word, it will send the messages to all dictators that there is no alternative but to remain in power.
This process has been followed in several instances. King Faroukh of Egypt was allowed to resign and go abroad with amnesty. Baby Doc of Haiti was allowed to leave with amnesty. If you don't honor the amnesty after a dictator has stepped down, then dictators will read the message and refuse to leave quietly.
I also happen to think that Pinochet did what was needed for his country. The same was true of Franco in Spain. In a revolutionary situation with civil war and violence on all sides, sometimes a dictator is the only thing possible. Franco, much as he has been maligned, stayed out of the war, kept Hitler out of Spain, took in refugees while France was sending them to the death camps, and left Spain a better place than he found it. Dictatorship is not the ideal form of government, but in some situations it may be the only available alternative for the time.