He can do it. Joker almost broke Superman's will in Action Comics. He fooled Kal-El into thinking the only means of saving a poisoned Lois would be to kill him(joker) so he can find out the cure. Had Batman not convinced Supes; Clark would have killed the Joker out of desperation. Had Supes done it he would have broken his oath of not taking another life and Supes' would have been broken. They found out it was all a farce after Lois made a complete recovery after Supes decided he would not kill Joker.