The horse killing malus, although good on paper, doesn't seem very natural. It could lead to combats looking weird / being blockfests because nobody wants to hurt the enemy's horse. More often than not we kill the horse in battle because it's a way of circumventing a block, or because the effect is similar to killing (when you are very far from the action for example)
If the current rules don't work well, I propose :
1° Fight on horses with the same rules except you get no penalty for only killing the horse and a rider kill gets you 3 points.
2° If one team isn't all dead (some riders were dehorsed without dying), a melee fight happens with all the alive players. In that phase, a kill is only worth 1 point.