Critias (460 BC-403 BC) was an Athenian poet and playwright. He was a follower of Socrates, and an uncle of Plato. He was also one of the so-called Thirty Tyrants, a pro-Spartan oligarchy installed in Athens after its defeat in the Peloponnesian War in 404 BC. Critias established a reputation as being the most violent and vindictive of the Thirty. He was killed when pro-democracy forces were able to drive out the Thirty.
Kritias (460-413 BC) was a follower of Sokrates. He questioned Sokrates's decision to accompany Alkibiades' expedition to Sicily, but went unheeded. When the triumphant Alkibiades returned and conquered Athens, Kritias publicly denounced him as a tyrant. Men loyal to Alkibiades murdered Kritias on the spot. Kritias' nephew, Aristokles, pledged to avenge Kritias, and was also murdered.
Kritias opposition to Alkibiades had little to do with the former's devotion to democracy. Rather, Kritias was jealous of Alkibiades' accomplishments, and had envisioned himself as a dictator of Athens.