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Solon seems quite certain that Starbuck is guilty, as everything appears to add up. Cassiopeia and Solon, at different times during the trial, attempt to get Starbuck to plead that Ortega's death was the result of self-defense, given that Ortega's weapon was drawn.
However, Starbuck chooses not to accept the plea. A self-defense plea would boot him from the military just as would a life-sentence.