I've seen a lot of theories centering on the children's not being harmed. Custody, wanting the kids not to grow up in criminal environment, avenging past abuse, cleansing the town of criminals, etc. IMO, there was no "noble" motive behind the murders. It's a common literary trope, but in real life, it's very, very rare for people to kill for their ideals (excluding honor killings in the Middle East, I suppose). These murders will come down to some sort of physical, tangible gain for the perpetrators - IMO, money, business, or elimination of threat.