As one who has been kicked several times, I can tell you that it hurts like hell and can be life threatening. Almost always, though, horses kick out not from malice but fear. Sometimes they kick out of exuberance such as when they are turned out. Sometimes they kick at other horses and people get in between.
Horses are magnificent animals but they have the brains of a duck, are creatures of habit and above all are herd animals. I always try to keep these truths in mind when around them.
My gelding nailed me good right in the elbow, turning him out one day. Fortunatly, it was winter, and I had thick clothing on. The kick did not break any bones, but I was still off work for a month, and had to have therapy.