In 'The Presige'....?

Cutter wasn't in on Angier's trick so he didn't know the tank was intended to lock. When he bursts in on Borden under the stage, he incorrectly umes that Borden locked Angier in the tank. Borden trying to save Angier could then appear as an attempt to look innocent once he has been caught in the act. Angier is the only one who knows how the trick works and he isn't at the trial obviously. Cutter was very loyal to Angier and aware of the rivalry between Angier and Borden so naturally he umes that Borden was trying to kill Angier. Since he believes that Angier is dead, the only alternative would be intentional suicide, which is apparently inconceivable to Cutter. His court testimony is colored by his umptions, his dislike of Borden and his upset over Angier's death.