People wouldn't be deterred from getting revenge.
Think of how strong the pain is of losing a loved one, and knowing the the killer might live right next door. Do you think the thought of jail or even death would deter someone who has just lost their entire family? At that point, they wouldn't even care about their own life, or societies laws, they would just want revenge on those responsible.
The violence of the purge wouldn't stop with the sirens, it would probably continue into complete anarchy or even civil war!