What finally makes the barber decide to let Torres walk away unharmed?

While lathering Torres, examining his beard, and listening to him explain how many rebels he has killed and is planning to kill, the barber has a conversation with himself, taking note of the ease with which he himself (a rebel supporter), could eliminate Torres with his blade. It is a life-altering decision, and while he contemplates it, he thinks of how nervous he is, all while outwardly appearing professional and carrying out his shave to a tee.The barber's internal dialogue, in which he weighs the pros and cons of killing Torres, is what leads him to the conclusion:

Just lather, that's all. You're the executioner and I am only a barber. Each person has his place in the scheme of things.

If not for the barber's love of the profession and his lack of experience with killing, we might be reading an entirely different story, one in which the barber could join the bloody revolutionaries and rid them of Torres. However, the barber is concerned about the scene all the blood would cause. In the end, his career as a barber is what saves him:

A blade like this doesn't fail. It is my best. But I don't want to be a murderer, no sir. You came to me for a shave. And I perform my work honorably. . . . I don't want blood on my hands.