I would just think of them as "different" and would not try to discipline them in the same way that "normal' people can be disciplined. That is the mistake of the parents... psychopaths will often not be helped by therapy or take it seriously, would be made worse by being forced to subject to harsh authority at boot camp, etc. The parents were making it worse, not better. Ask any psychologist that has studied it- there are certain ways that you have to deal with psychopaths because of the way their brain works. They can be manipulated, taught, and made to self-control just like any child (in many cases- I guess some may be lost causes anyhow) but you have to know what they need and how they think. In many cases, the best thing is substitution. For example, you know how kids have fake versions of things so they don't try to play with the real thing? Like a toy kitchen or easy bake oven so they don't try to use the stove? Likewise, you have to identify what behavior of the psychopath is really a problem, and give them another (better) way to satisfy whatever urge or need they are feeling that causes them to do it. In the case you mention, maybe they could teach the kid to hunt properly because he is killing animals anyway?
I certainly don't think it was excusable to kill their child because of that. What if your child were autistic, and you tried to discipline them into being "normal" but their thoughts and behavior didn't change, so you killed them for being born that way? Awful, right? To me, it's the same thing to kill a psychopathic child just for being the way they are. They didn't ask to be that way.