Life really can change in the blink of an eye

I just really don't understand people. Violence just seems so unnecessary. There I was sitting eating dinner with a few old friends when one gets this god awful news. Domestic violence... but what exactly happened? Of course we ended things right then and there. She was off to be by her husband's side who must be completely hysterical. Why do these things happen? The last time we saw these two people was such a joyous occasion. Everyone was laughing and seemed genuinely happy. What makes a person, who days earlier was smiling and in great company, pick up a gun?