So, Nozdormu could live forever, splitting off timelines and other timey-wimey stuff... but he wouldn't be able to live with himself. His personality, standards, morals, emotions, intelligence - Nozdormu in toto - that is what makes him mortal. He cannot act against himself like that. No one can. So in other words, even though he knows the events surrounding that death, he cannot change it because to do so would be to deny everything he is.
Anyway, Murozond is from a separate timeline, I think...?