I'm talking about an immoral but likely effective policy, where whenever there are Kamikaze mass murders (like the OP), the mass murderer's family is subsequently punished. Innocent people (just like the mass murderer's victims) would be put in jeopardy with this policy, but it might be just the right innocent people that would deter these scum of the Earth mass murderers. You make an imaginary scale or balance. Today, when the mass murderers consider their crime, the scale has his murderous desire on one side, and the lives of unimportant people on the other. No contest, his desires outweigh these innocent people.
Now put their family on one side of that scale, with this immoral policy I'm not seriously entertaining. Now that stops some of these Kamikaze mass murderers. They weigh their bloodlust against their family. Is it going to stop all of em? no. Some? Many? Yes.
But it's still an immoral policy. So even though it's possible to argue that it would probably work, it's still off the table because it's immoral.
Like gun control.