To answer this question, take a look at what the barber says on the subject of killing Captain Torres:
I could cut this throat just so, zip! zip! I wouldn't give him time to complain and since he has his eyes closed he wouldn't see the glistening knife blade or my glistening eyes.
In other words, Captain Torres has no idea that the barber is helping the rebels. In fact, he thinks that the barber is just a barber, so he has no reason to expect that he would be murdered while having a shave. As a result, Captain Torres would not have the opportunity to defend himself against the barber's razor. He would have no warning of the violence about to occur. Moreover, after killing the captain, the barber would be forced to flee and hide because the captain's men are sure to seek revenge.
For the barber, then, murdering Captain Torres would be a cowardly act. This idea contributes to his decision not to murder the captain. In addition, the barber is proud of his profession, abhorred by the notion of murder, and simply wants to be seen as a "revolutionary," not a criminal.
Killing a man by slitting his throat while he sits before you with eyes closed seems to be a pretty cowardly act to me. Accepting money for a service and then committing the act only makes it more shameful. When Captain Torres came into the barber shop for a shave, he probably knew that there was a possibility that he would not leave alive. But he must have judged the barber correctly: If indeed he was one of the rebels, the public murder would not only blow the barber's cover--thereby rendering him useless for future information--but also possibly turn public opinion against the rebels for committing such a murder. Few acts of murder can be called anything but cowardly, but the method that the barber would have been forced to take--cutting the throat of a defenseless man from behind--would be hard to defend.