This is a difficult question, as really, there seems to be no firm evidence either way to suggest that the Misfit would not have killed them had the grandmother not recognised him. What is interesting though is that after the identification, the Misfit says something that is rather intriguing:
"Yes'm," the man said, smiling slightly as if he were pleased in spite of himself to be known, "but it would have been better for all of you, lady, if you hadn't of reckernized me."
This seems to suggest that if the grandmother had not identified him, the Misfit would have let them go alive. And yet, I am forced to wonder, if this is actually the case. The Misfit and his gang are clearly after another car. Because they are on the run, they would not want anyone left who had identified them and who could give the police any information about their new set of wheels or their direction, and so it seems clear that the Misfit would have killed them anyway, if you consider the argument from this perspective.