A war criminal is someone who is accused of committing a war crime.

In an alternate reality, Nero was considered a war criminal, after he destroyed the planet Vulcan and caused the deaths of six billion people. (Star Trek)

Gul Darhe'el was considered by many to be a war criminal. (DS9: "Duet")

Worf and Ezri Dax were to be charged executed by the Cardassians for being war criminals after being taken prisoner by the Breen in late 2375. (DS9: "Strange Bedfellows")

Dukat was accused of being a war criminal for his actions during the Occupation of Bajor. Kai Winn Adami once told him that she didn't forgive war criminals. (DS9: "Waltz", "What You Leave Behind")