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!