Dahmer killed one person at a time. He didn't shoot his way into a school with a military style assault rifle. You are right. There is no way to prevent evil people from doing evil things, but you can throw obstacles in their way. That's all we can do besides wringing hands and gnashing teeth.
And, no, money won't bring back dead children, but it might help defray funeral expenses and rebuild the structure that the shooter demolished in his frenzy to kill. All this chaos and destruction caused physical damage that will require labor and resources to put right. Liability insurance could have helped at least defray some of this. It's not much, I know, but if I have to have it in order to operate an automobile, is it too much to ask that owners of guns likewise be held responsible for accidental damage? It won't bring back Ms. Lanza, but at least it will insure that her estate will help make right some of the terrible damage wrought by her failure to insure that her guns were secured and kept safely away from a mentally defective son.