Masterson demonstrated no mercy in raping his victims.
Our society demonstrating no mercy to those our society failed, and we did at some point for most felons if not Masterson, just makes us another link in the chain of cruelty. Mercy should always be considered, at every step, especially if we as a society espouse to be better than those we prosecute.
Justice isn’t for pleasure, vengeance is. If you’re feeling good about anyone’s suffering, Masterson or his victims, that isn’t a sense of justice you’re feeling.