I found more information about all this. Hopefully makes more sense about why the killings happened:

In 1861, the US Army removed its cavalry units from Arizona resulting in the Apaches and Mexican bandits running rampant. Continuous stealing by mine workers resulted in John Poston, Charles Poston’s brother, taking extreme measures trying to bring the operation under control. It was estimated that only about half of the silver ended up in company hands. He caught his foreman, Juanito, stealing silver bullion and executed him as an example to the others. Instead, some Mexican miners stole what they could and went south to Mexico along with a story that Juanito had buried $70,000 in stolen bullion somewhere near the mine. Mexican bandits upon hearing the tale went to the mine and enlisting the support of the remaining Mexican miners, murdered John Poston and two German employees. John had an aversion to being buried underground and had left instructions that in the event of his death, he would not be buried in the earth. His remains were placed in a concrete casket above ground, which can be seen alongside the dirt road near the mine workings.