As a newspaper reporter, I saw or heard of many, many horrible stories involving guns shot accidentally or in a moment of stupidity. I got to know a family where the brother accidentally shot a gun and killed his sister. I knew of a 10-year-old boy who shot and killed his twin brother during an argument. A father who shot and killed his 14-year-old daughter when she hid in the house and then jumped out to surprise him. A (really foolish) mother who pointed a gun at her teenage son to get him to clean his room and accidentally shot and killed him. A man who shot and killed a Japanese exchange student in a Halloween costume who was coming through the yard to visit his son.
So right there we have five young people who'd be alive today if guns hadn't been handy.
I once saw a really good bumper sticker: "If guns are outlawed, only outlaws will shoot their children accidentally."
There's another saying that gun proponents like: "Guns don't kill people, people kill people." True, humans are violent. But when people are restricted to using knives or clubs or whatever, the damage they can do is much more limited than if they have guns.