Courts continue to handle sexual predators (chiefly pedophiles) with kid gloves. They set them loose with barely a slap on the wrist and mere months to a couple of years later, they're molesting someone else's child again. It's almost as if the courts are passively condoning it. Once a pedophile offends, you can't rehabilitate that, and certainly not with house arrest. Honestly, if the system won't handle the situation, I'm not mad at all for society and/or the victims' loved ones handling it themselves. I'm surprised these crimes of passion don't happen more often.
Problem is as you can see in this case, innocent people then get caught up in this kind of vigilante justice.
Like you said, I understand Cain reacting in this way. I get it but it just isn't worth it in the long run.