Choice

Within the Doctor's time stream, the "ghost" of the War Doctor told the Eleventh Doctor that he did what he did "without choice, in the name of peace and sanity." The Eleventh Doctor countered "but not in the name of the Doctor." (TV: The Name of the Doctor)

Behind the scenes
Reflecting on his participation in the Time War, the War Doctor mentioned that he never wanted to fight, but had no choice. (NOTVALID: He Who Fights With Monsters)