Is it OK to assassinate a dictator?

No. That is murder. You would be no better than the dictator. A dictator can be overthrown by force, but a peaceful humane regime must take over. So if the dictator can be overthrown without being killed, they can be brought to justice. The dictator and his key assistants who have been involved in any of the crimes of the Dictatorship can all be imprisoned. All of this would be a sign of a new positive future for a country, a complete change from the way the country had been run until that time.