In the area I grew up in there were two guys who had been best friends since early childhood and were essentially like brothers to one another. One of them met a girl and fell in love, but for some reason he got it into his head that she was cheating on him with the other friend.

So, one evening in their regular pub he confronted his friend and punched him in the face, sending him falling backwards and he hit the back of his head on a table and died instantly.

One punch and he killed his best friend who was completely innocent, and, of course, he went to prison for (I think the charge was) manslaughter.

That story has always stayed with me and I wish more people would realise it only takes one punch, one punch to kill someone and ruin your life.