The only real issue I have with this event is the guy didn’t return to euthanize the horse. Latches can get forgotten, some latches can bounce unlatched. However, once those incidents happen I believe we have a duty to act as quick as possible to end the animals suffering if his injuries are beyond repair. I’m thinking there’s a little more to this story possibly. I don’t always have a gun but I always have a knife. Any horse person, cowboy or farrier should have a knife. Being around horse and cow stuff a knife is a top priority. I’ve been part of one horse falling out of a trailer and get drug for a ways down a gravel road. The horse was back in the trailer before we could get out of the truck after stopping. The horse lived and healed up plenty good to be used as good as ever. He did have some wild scars on his front legs. Anyhow, I’m wondering if this guy was intoxicated and wanted away from the scene.
Only thing they might charge him with after the lawyers get involved is probably littering. 
I have had people open the rear gate of a stock trailer full of cattle while siting at a red light. I now chain and padlock the rear gate.
I have the fear of what you described every time I stop at a light. One of my biggest fears is someone opening trailer doors while the rig is sitting in a parking lot and we are inside eating lunch. If we have multiple vehicles we will pull another truck up to the back doors. Or I’ll back the trailer up to something that blocks the doors. I most always try to find a window seat while in a restaurant. A chain and lock would solve this but animals in a confined space behind a lock make me just as nervous.
