There are two separate issues here, Plato's specific criticisms of the Athenian democracy and Plato's attitude towards democracy as a system.
Plato was far from the only person to have concerns about the Athenian democracy. Many other writers portray the demos as being easily swayed or even bribed by demagogues, and prone to factional strife (stasis), violence, irrationality, and a sort of moral blindness. It was the democratic faction in Athenian politics that supported the Peloponnesian wars, senseless conflicts that resulted in a huge death toll and the eventual fall of Greece to Macedonia. The democratic faction also wanted to prevent the Melians and Mytilenians from remaining neutral in the wars by enslaving all the women and children and killing the men. Perhaps most importantly, it was the Athenian democracy that killed Socrates.
On a more philosophical...
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