In the golden era of ancient Greece, a medley of traveling teachers specialized in rhetoric and the philosophy of science. Rhetoric was a new skill made necessary in a democracy which remains mostly misunderstood to this day. In ancient Greece, these poor vagrant teachers, eking out a living by teaching children how to rule, were called sophists. Here is the most famous of them. Socrates, in his one suriviving portrait in Turkey near Ephesus (apparently drunk and with devil's horns).

The Only Portrait of Socrates
The Only Portrait of Socrates

Socrates was the most famous sophist, due to some idiosyncrasies such as refusing to wear sandals (shoes) and also, his denial that he was himself a sophist.

Plato assembled the sophist rhetoric into one volume in the form of very well written plays, with Socrates as the protagonist. Socrates became much richer than other sophists as a result, and even purchased his own boy slave as a sex toy. This was not unusual at the time. Most Athenians who could afford young boys as sexy toys owned at least one. 

Other Athenians objected to his popularity and fiscal success, which was atypical for teachers at the time, and attempted to have him exiled, under the excuse he was an atheist (which had previously not been a crime, and was probably the worst direct consequence of his life), but as any politician would tell you, more because reasoned thought undermines the power of those who manipulate power for their own success. Even president bush Jr.called academics 'elitist; and therefore despicable.

Exile in the Hellenistic era meant living in the most barbaric conditions somewhere in Albania or worse, and Socrates committed suicide rather than attempt to teach outside Athens again. Plato described his suicide as absolutely noble, and it was probably a main inspiration for how Jesus later decided to conquer the Romans through moral sacrifice.

Later, sophists built an actual building for themselves which was Athen's first university. But in around 300AD, it disappeared mysteriously. by reading Proclus, it appears to me the school secretly joined the Christian church and invented the dark ages, that is, that the common person cannot understand the true meaning of life and needs to be held in awe of a supernatural force known only to a privileged few, and that this method should be secret.

This was based on the theory of Nazism as the best form of government. If you have thought though the rhetoric, you will clearly observe that Plato invented Nazism,. He defended it in his book 'the republic.'

So Plato can thus rationally be held the route cause of the dark ages, which caused millions of people to suffer for over a thousand years.
Plato also established the basis of Hitler's power. Hitler is probably the most famous user of Platonic theory. 

So in sum, besides making Hitler and the dark ages, Plato was ok.