I've read the first book. In my mind, it would have been morally justified for almost anyone else to do it. Aerys was a horrible king who needed to be killed. However, Jaime took an oath to serve him. Thus, Jaime being the one to kill him was not okay, particularly because he did it only to put himself ahead not because of any desire to do good.

I know that later on, GRRM tries to redeem Jaime but doesn't do the same with Cersei. That pisses me off, actually. She's the one who is more sympathetic, considering she is a woman in a society that treats women horribly and is stuck in a marriage with an abusive husband who flaunts his cheating in front of her.