It's hard to know but, if the colt was seriously injured/pts some people would be more than capable of asking you to cover the costs! If the owner is that type then there's a chance that the colt is actually ok.
I most likely in that quick thinking action needed scenario, would have done the same. To protect myself and to protect my horse.
Not the same I know, but I have had two snarling aggressive dogs lunge over a fence at her face. I walloped the pair of them until they stopped- and I mean walloped.
I informed the owner who put up a slightly higher fence. We didn't fall out over it.
So I reckon based on that response I would do the same to a horse that was downright dangerous.