ACTUALLY Martha isnt reaally why Batman stopped himself from killing Superman. In fact it was the symbolism of finding oneself in the role of the monster which haunts their life and essentially their soul. For that is why batman saw manbat, a creature who is a twisted and inverted representation of batman. Manbat allowed the BAT to overpower him.
Anyway. Martha and what she stands for is a thing of debate im sure but you must see all the subtle cues from snyder early on. Ever wonder why thomas says martha in the opening scene?
Its all clock work, by the artist Snyder to show us a man is only a man if he is a Batman. A batman who can only love himself and not one who can love those who have loved and lost but never loved again.