We doubt if we will ever get a definitive answer, especially one that satisfies all people, or even most people. As God Himself says in Genesis 8:21, “The intention of man’s heart is evil from his youth” (all of the mass shooters seem to be men).
However, we urge everyone to consider this answer:
By removing God, the Bible, God's Law, Jesus Christ, and the Holy Spirit from society, including the mass media and the schools, we are raising generations of people with no faith in God or Jesus and, hence, no moral conscience, and no self-control. If so many people have no faith, no moral conscience and no self-control, then it’s no wonder our society is suffering from all these mass murders by evil lone gunmen.
For some reason, the word “evil” has become a pariah in modern discourse. If you remove God, the Bible, God’s Law, Jesus Christ, and the Holy Spirit, then doesn’t the notion of good and evil become meaningless?
As Paul points out in Romans 2:29, people become good and do the right thing only when their hearts, or their conscience or inner souls, are at least partly sanctified, cleansed, redeemed, and transformed by the power of the Holy Spirit.