Each and every homicidal person gave numerous clues that something was up. Red flags of you will - each of which were disregarded. Since society cannot be made foolproof or completely safe (increasing stabbings and other assaults in Japan, China and Britain), focusing on behaviors which hint of things to come can be paramount.
An ounce of prevention is worth a pound of cure.
EDIT: With rare exception, each of these individuals is SUICIDAL. The suicidal through process cannot be separated from the HOMICIDAL thought process. Is anyone asking why so many are suicidal? Why so many want public attention for their acts? Why suicide is the act of choice, carrying homicide with it?
We have assisted suicide, euthanasia, abortion and ever decreasing penalties for homicide. Human life, as the Church teaches, has been devalued on all fronts. So, like an empty McDonald’s drink cup, it has become disposable. We are fighting culture rot and the solution is not more and more legislation and - we have tons of laws already. The solution is in the human heart and head. Mental health has been ignored for decades.
As a society, we reap what we sow.