May a defendant claim defense of a third person without testifying?

If the defense could present sufficient evidence of the threat to the third person, defendant's state of mind, need to use (deadly) force in defense, &c., then yes. I had a defense attorney attempt this in a trial a while back, with no success (not enough evidence of D's state of mind and need for deadly force), which drove the defense attorney mad, since he waited until the charge conference to deal with the issue. But, I would also argue that the defense has failed to admit to committing the offense, which triggers the justification defense.

-Joseph