Just a few comments. I don't have huge experience with horses, so you can decide if they are relevant...
1 - I've frequently met DOGS who were aggressive toward other dogs, or people, but not both. I've owned a number of dogs who would give their lives without question fighting a dog to protect their people, but who would rather die than ever hurt a person. A 65 lb shepherd mix is now living with my oldest daughter. He is an outstanding guard dog, and he once placed himself between my wife and an aggressive 180 Rottweiler in a stance that clearly said, "You'll have to kill me first" - but if a baby pokes him in the face, he just hides his face.
If horses treated people the way they treat themselves, we would never be able to ride. They would just beat the tar out of us and go back to eating.
2 - Some horses develop hatred for another horse. I sold a mare in Dec 2010 because Trooper hated her, and she hated him. And it was only getting worse. Both got along fine with other horses, but would look for a chance to attack the other one. Trooper was heavier and could win, but Lilly developed a dislike for him that turned into mutual hatred.
She was uncommonly good with every other horse she met, and is now sharing a corral with 2 other horses. We share a trainer & a farrier with the new owners, and I'm told they think she is the best horse they have met. And without her, Trooper does well.
3 - That said, I'm not big on taking chances. Last December, we were given a mustang pony. Small enough for a child to mount, but strong enough to gallop easily with my 175 lbs on his back. He is sure-footed and level headed. The equine dentist, after working on his teeth, offered to buy him on the spot for her nieces. And if I wanted a 4th horse, I'm sure I could have another one free within a week.
If I had a dog that was truly aggressive toward other dogs - who would seek them out to kill them - I'd shoot him ASAP. I've known people who keep dogs like that for years because they were good around people, but I'm not one of them. I ended up with a 3rd dog after one followed my brother-in-law home from the desert. The dog is playful, very loving, and is the gentlest dog I've ever met around babies. Why would I want to take a chance on a dog that kills other dogs - even just once?
I feel the same about horses. If it was a valuable stud, I might put up with it. If I was in to a particular sport, and the horse was awesome at it...I'd figure out how to deal with it. But for just riding? Not a chance! I wouldn't know how to live with myself if there was a repeat killing...