Democracies have no preventive measures in law. If someone comes to your house and points a gun at you and says he is going to kill you, you have very little legal recourse. He will be arrested once he kills you but may be acquitted in a trial.
It is no different in a Democratic Israel. the perpetrator(s) will be arrested. The Israeli Prison system just released 100 terrorists for lack of space. The Middle East is not a democracy and there are incentives to kill. Vigilante justice creates fear and does a lot of damage. Almost none have been killed contrary to MEMO.
This is not per se vigilante justice and in the long run it will work or we will have a revolving justice system and more maiming and murders. Democracy is not a suicide pact. When in Rome do as the Romans do.