Does a horse freak out if it steps on a rabbit and kills it?
A rabbit is a pretty small animal and a normal/large horse may not even be aware that anything has happened. If they do realize it, they won't like it but unlikely to "freak out". But... after 40+ years of riding, training and falling I have realized that horses are extremely squeamish about stepping on a live body, human or otherwise. When you fall, they will almost always do just about anything to avoid stepping on you. Same if a dog or cat get under their hooves. Human feet don't count because most people wear hard boots/shoes and it doesn't have that squishy feeling that a body does.
It depends on the horse … some horses could have a hissy fit - where other horses may not seem to even notice.
Not at all. Some horses will aggressively chase smaller animals and trample them to death.
Yes it would be very upset.
Yes it can. But it depends on the animal. Some horses like to kill things others do not. Occasionally horses will eat flesh and drink blood.
Yes. It is the worst possible thing to happen to a horse apart from being maimed. It’s an experience that haunts them for the rest of their life.