Diodotus, son of Eucrates
was an opponent to the proposal of Cleon

Cleon

Cleon was an Athenian statesman and a Strategos during the Peloponnesian War. He was the first prominent representative of the commercial class in Athenian politics, although he was an aristocrat himself...

 - leader of the aggressive faction in Athens - in 427 BC to kill all adult Mytilenean males and to enslave their women and children after the defeat of Mytilene

Mytilene

Mytilene is a town and a former municipality on the island of Lesbos, North Aegean, Greece. Since the 2011 local government reform it is part of the municipality Lesbos, of which it is a municipal unit. It is the capital of the island of Lesbos. Mytilene, whose name is pre-Greek, is built on the...

  (see also Mytilenean revolt

Mytilenean revolt

The Mytilenean revolt was an incident in the Peloponnesian War in which the city of Mytilene attempted to unify the island of Lesbos under its control and revolt from the Athenian Empire...

). He represented the moderate faction in Athens (in favour of Pericles'

Pericles

Pericles was a prominent and influential statesman, orator, and general of Athens during the city's Golden Age—specifically, the time between the Persian and Peloponnesian wars...

 policy).

Diodotus' proposal won in the assembly, so that in the end only Cleon's next proposal was executed: to punish by death those Mytileneans who were sent by Paches to Athens (which were a little over a thousand; this was probably about 10% of the adult male population of the rebelling cities on Lesbos). The execution took place without proper trial.

Diodotus' one speech in Thucydides is all we know of him.