Multiple realities(covers information from several alternate timelines)
A war criminal was someone who was 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)
Khan Noonien Singh and his fellow Augments were condemned to death as war criminals, according to Alexander Marcus. (Star Trek Into Darkness)
Gul Darhe'el was considered by many to be a war criminal. (DS9: "Duet")
Worf and Ezri Dax were to be charged and 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")