The key word I think is relative innocents they are not truly taking justiceinto their own hands and deciding that that is is the law .They are following the law if the most extreme interpretation of it. Even Jasnah killed those who were guilty of committing awful crimes. The Sons of honor make no qualms about killing those who have literally done nothing so long as it gets them to their goal .That is a stark difference between what the sons of honor have done and what the others have done you may be able to get away with doing the wrong thing, but we have yet to see any who have been accepted and were as indescriminate as they are. They may also keep to an "ideal", however that may not be enough to earn them spren considering the severity of what they do and in fact the despicable nature of their goal.