Read up on exactly what Mugabe is doing
This idea that a group of civilians can enact violent revolution hasn't been valid for the past few decades. Dictators tend to have a monopoly on force in a country, they command the loyalty of military and police forces inside the country, and remain in power because no other force in the country has the capability to oust them.
I.E. Even if there were free elections in Zimbabwe, and the people DID elect someone other than this monster, he could simply say, "No." and carry on with his business.
Naturally, there's the flip side of this, foreign interference producing a government even WORSE than the last one. The list of Western-inspired coups that produced shitty governments in the end is pretty long, but without foreign assistance, rebel groups don't really stand a chance to create meaningful change.