If Jaime had not killed Aerys II, then either the city would have been blown to bits or Ned would have confronted Aerys himself. What would have happened when Ned faced Aerys? Would Ned have killed him in retribution for his father and brother or would he have restrained himself and given Aerys to Robert for judgement?
I wonder about Ned's honor if he'd had that decision to make. Imagine it... the middle of a war... the prize is in sight... you are leading the vanguard... swords, blood, fire... death is everywhere... you are losing men to the Mad King... and suddenly the murderer is there... a madman is gibbering on the Iron Throne, the man who killed your father, the man who killed your brother, and the man whose son has raped your sister... the very man who ordered your death six months ago, and he'll kill you now if he gets the chance... do you give him over to Robert? or do you gain justice for your family... you feel the greatsword in your hand... it calls to you "Ice can quench fire"... the Valyrian blade can end the line of Valyrian maniacs... what do you do?