The poem is a short, dark sketch of people who hold power over others and abuse it. The "masters" take something small, a minor performance, perhaps a mistake or a modest act, and blow it up into rage. The poem never says who they are or who suffers, and that vagueness lets the idea apply to any setting where one person has authority over another.
The second half is the most unsettling part. The rage becomes something the masters "nest in," as if it were a home they settle into and feel comfortable inside. They "sing behind it," which suggests they hide behind their anger while pretending, or even enjoying, that it is something else. Then they "write over it in blood," a violent image of erasing what was there and replacing it with harm.
The speaker doesn't shout or lecture. The tone is cold and observant, and that restraint makes the mood feel tense and ominous, like watching cruelty settle in and take root.