Why did Artemio Cruz kill Pedro Menchaca?

As a boy, Artemio is almost totally neglected on the plantation. As the illegitimate son of Atanasio, one of the masters, he had no proper place in the hierarchy; his father had wanted to kill him. His mother, a biracial servant, was physically forced to leave the estate after his birth. His sole protector is Lunero, a black servant who has assumed responsibility for most tasks on the plantation because Pedro, Atanasio’s brother, is debilitated by alcoholism and other diseases. As the situation grows ever more desperate, Lunero decides to leave to find work elsewhere. By then, Artemio is basically old enough not to be a liability. Artemio overhears the old mistress (his grandmother), Ludivinia, talking with Pedro about their plan; she orders Pedro to stop them by any means necessary. The boy takes Pedro’s shotgun down from its place hanging on the wall and hides near the road with Lunero. When Pedro rides past, Artemio shoots him so he cannot prevent their escape.
https://archive.org/stream/in.ernet.dli.2015.126611/2015.126611.The-Death-Of-Artemio-Cruz_djvu.txt