Reasons people shoot other people:
My fiancee and I had an argument, so I open-carried my gun to a park and shot four random people.
The bartender put Clamato in my beer when I wanted tomato juice, so I shot him and his dog.
I found suspicious calls on my boyfriend's phone, so I shot him. He was armed at the time too.
Rather than let my ex-wife win custody, I shot my own daughter to death.
Your Second Amendment-guaranteed well-regulated militia in action. Those above were just from May of this year, and it's only a sampling.