Spain's a puzzle. They tried desperately to get a republic going at least twice (starting in the mid-to-late 19th century) only to fall back into old habits of bloody civil war. Franco was the last of the dictators, ruling the country with an iron fist for, what, 40 years? Then he dies, everybody heaves a sigh of relief, the kindly old doddering monarch comes back from exile, and just like that Spain joins the rest of western Europe in a perfectly civilized constitutional monarchy. Where the heck did that come from?

One route to democracy is the one Dan mentioned in his comment, a gradual evolution over centuries with a larger and larger percentage of the population getting a political voice (along with a continuing extension of basic human rights). England followed this route, as did the Scandinavian countries.

Spain does not follow this pattern. It's Spain that made me ask if there might be more than one set of prerequisites for democracy to take hold. Hypothesis: Franco was the key. He wasn't exactly a nice person, but he wasn't a monster either. He had no qualms about killing anybody that opposed him, but once he solidified his hold on the country he just settled down and ran the place. It might be a stretch to call him a benevolent dictator, but he was at least a non-malevolent one, especially compared to some of Spain's historic monarchs. He lived a long time, helpfully spending the last few years in a coma, which gave the country a chance to transition gradually to life without Franco.

Truly malevelent dictators leave nothing but chaos behind them; the Duvalier family of Haiti being an excellent example of this.