OK. The law in my state is that cruelty means that somebody intentionally causes suffering or tortures an animal. If CO just says “needless” killing then so be it. Its a bit abusive of the intent of the law, but there it sits.
A decent attorney will beat it. This douche was acting for a reason. It was illegal and wrong, but it wasn’t without a reason. He was trying to keep the bison off his property. That’ll be enough to beat that rap with a jury. But there’s plenty left to nail him with that WILL stick.
Personally... I like the bowie-knife solution you mentioned above. :-). Simple and effective.