Within the framework of O'Connor's Christian concept of "grace," that a divine pardon is granted by God simply for the asking, the Misfit must be the one held morally responsible for the killing of the six family members. For, it is the petty, selfish, and forgetful grandmother who attains grace at the end of "A Good Man is Hard to Find" as, at the moment of her death, she reaches out to the Misfit exclaiming, "Why, you're one of my children."

In the context of O'Connor's narratives, grace is something often undeserved, a force outside of the character that generates an epiphany.  The grandmother has such an epiphany and receives grace as her spiritual blindness is removed.

I'll say Flannery O'Connor.  Ultimately, it is only she who controls the LACK OF MORALITY in her fiction.  She would not want us to choose either the Misfit or the grandmother since all her characters are doomed.

O'Connor once said:

This is a generation of wingless chickens, which is what Nietzsche meant when he said God was dead.

Her comic religious vision holds that a morally and socially degenerate (The Misfit) is nonetheless spiritually a cut above the wingless chickens of privileged Christianity (the grandmother and her family). She shocks her readers by beginning with divine evil as a backdoor to what is divine good so that they may rediscover what is holy. Her goal, I think, is to prevent her readers from taking sides among her religious forms; instead, she calls for action--from them to be seekers instead of being found.

When reading O'Connor's prose one can feel the laws of attraction at work: good begets good; evil begets evil. Syntheses and concessions are pitfalls. Either one is Christ-centered or hell-bent toward the fumes of the gas chamber. Her poles are distinct and opposing, the slippery slope a descent to hell.

So, you really can't judge any of her characters morally.  They are all morally irresponsible and, therefore, fated and doomed.  Like Kafka's, O'Connor's characters are caricatures, flat, static, submen as they populate a fallen, Christ-haunted world.

It would be hard to argue that anyone but the murderer himself, The Misfit, is most responsible for the killings of the family in "A Good Man is Hard to Find." After all, The Misfit and his accomplice made the decision to pull the triggers when they could have just let the family go on the isolated country road. The grandmother can also be held somewhat responsible for forcing her son to make the unfortunate turnoff in search of the misplaced old plantation home. Without this example of attempted self-gratification, the family would have survived to enjoy their Florida vacation. The father can also shoulder some of the blame for knowingly taking his family into the path of the killer.

Great question!  I believe the one who is the most responsible for the death of the family is the grandmother, and for two reasons.

One, it is literally her that gets the family in harm's way when she tells them to turn off the main highway to find the house she thinks is still there.  However, her memory is bad and it's not the way.  It only leads them right into the path of the Misfit.

Secondly, the line the Misfit says after he shoots her really sum up the psychological reason she's responsible:

"She would of been a good woman," The Misfit said, "if it had been somebody there to shoot her every minute of her life."

It was people like her that had led the Misfit to his life of crime---people who always preach about what is right or wrong, but their words are hollow because they don't live by the words they say.