Two tragedies produced by gun

A Korean student shot many people with his gun and killed himself in Virginia.
In Japan a yakuza shot Nagasaki mayor.

The Korean students was always lonely.
He had once written about death.
His professor thought he was dangerous and told him to have a counseling.
What made him do it?

The yakuza killed the mayor because he was he had something against Nagasaki city.

Why do people go violent so suddenly?