Why? "Why the generocity?" Calvera himself explains that he "doesn't think they'll return", but why not make sure, if life was that cheap anyway...
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OCD predator: www.goo.gl/0avZjB
Calvera isn't completely without honor. He is not a sociopath. He doesn't regard himself as a criminal, so much as a patrone who must do what has to be done to take care of his men, who are his family. He sees it as a business. He thinks of the seven hired guns as competitors he has beat in competition for the village, not as enemies.
Also, as he explains, if he kills them word will travel north of the border and others will come south to exact revenge on him, and he already has enough problems without having to deal with that.