Perhaps if Ben had not displayed new-leader qualities, then Locke would have been picked even after he failed the knife object test. Ben could feel regret as he knows deep-down Locke would have always been the better leader (his comments about seperating emotion and command for example). Therefore Ben may feel regretful he ever viewed himself as a leader, thus stopping Locke to be chosen and thus causing his 'misery'.
If Ben had never had crossed the security fence with the rabbit, then maybe Locke would have been chosen as the new leader without the need for an Alpert test?