Disclaimer, I had horses in my youth, however I am not an adult horse person.

The answer to a lot of your questions is that a reasonably well trained (and well tempered) horse should let most anyone ride it. Horses are pretty smart and can definitely get some anxiety if someone completely new just shows up and tries to ride them, but again, if they are well trained they know that they have a job to do and they do it.

I can't speak specifically about if a horse watched its owner die how it would react, so I'll lean into your western scenario. If someone walked up to me and shot me with a gun in front of my horse, and my horse was not familiar with gun shots, then the horse would probably get spooked and run away. Horses are flight animals and once they're spooked every instinct they have tells them to run.

As for telling at a glance how "obedient" a horse is, I can only say that I personally am not so skilled with horses that I could tell that. I can get a read on a horse's current mood based on how it reacts to me, but that involves interacting with the horse a bit and I can't say I'm perfect at it. Every horse has a different personality, similar to people.

Hope that at least gives you some answers!

Horse people?

In your Western scenarios, I don't think it would be anywhere near a clean getaway most of the time. Horses do tend to behave differently for different riders. A very well trained horse may handle the new rider better, but most horses tend to test new riders. The first couple rides on a new to you horse is both a learning experience and a power struggle. You don't know exactly how that horse was trained, and the horse usually wants to see what it can get away with. You might find out the owner doesn't use some standard cues, and the horse might just take advantage of it get irritated with the confusion.

They know who their people are. In your scenarios, it's likely the horse only has one rider. Trying to hop on without it's person there is going to confuse it. If you are good with horses and it's trained in a conventional way, you'll have a better chance at gaining control quickly. I still wouldn't imagine galloping off immediately... That's going to take time.

I really don't know with the shooting dead question. If owner does a lot of yelling before/after the shooting, I don't see it ending well. No arguing before hand, one clean shot to the head - might be more dependant on if the horse is gun trained. I know horses that couldn't care less about fireworks and gunfire but will throw you over a plastic shopping bag blowing across the field or because she heard a noise coming from "that scary tree".

I don't know a good way to judge a horses temperament at a glance. You can get an idea of what kind of mood they're currently in fairly quickly though. You can look at their muscle build and get an idea of how much they're worked and if your good an idea of what kind of work. Different muscles will be toned differently depending on what they're used for.

Hope that helps!

I don't know how to steal an entire horse. But if you praise one, bring it snacks, take it for long walks on the beach, maybe read it some French poetry under the autumn moonlight... well you might steal its heart.

I only know a few smatterings of horse facts having spent just a bit of time around them (let me know if you know Bulldog Hanover) but I can judge the basic mood of a horse at a particular time.

The danger sign is when its ears are fully back, means it's stressed, pissed, or scared. Likely to kick, buck, try to bite or be disobedient around a stranger trying to ride away on them.

Ears perked up facing forward (known as pricked) generally means the horse is alert, attentive, worried or curious about something in its surroundings. Hearing a nearby gunshot might startle and tense a horse up and prick their ears toward the noise, or hold their ears out to try to figure out the noise and why it was so loud.

Ears up but facing out in either direction is a horse's neutral, calm and relaxed position. In this state it would be the best opportunity for a stranger to get a ride from the horse. Doesn't mean it would accept giving a ride without issue, each has its own temperament and training of course.

Source for a full list of typical ear positions

We owned horses for years. Horses aren't really like dogs. If the horse rider is experienced, and the horse is already saddled, then it'll most likely be successful. It has more to do with the rider's experience than the horse's familiarity.