I live rurally and have friends and family who are gamekeepers, farmers, gun owners etc. This isn't a countryside v town issue though. Doesn't really matter if you hate or love foxes either. We have wildlife laws and the QC may have broken them.
For example, a man who legally trapped a grey squirrel (an invasive pest species which it is legal to kill) was prosecuted and fined because he despatched the squirrel by drowning it. The court said that caused "unecessary suffering" to the animal which is prohibited by the law.
Unless a vet euthanises it, the law pretty much requires a creature be shot or be despatched by a SINGLE blow to the head. That's possible with something like a pheasant or squirrel if you know what you're doing, it's unlikely with an adult fox and a baseball bat.
In fact the RSPCA has previously said that as most people do not know how to kill an animal without breaching the law they recommend calling them, a vet or pest control experts to do it legally. When it comes to injured wildlife, some vets will do this for free (as one even confirmed on the QC's Twitter thread).
I gather too that some people with chickens keep falconry gloves/motorbike gloves and pliers to disentangle wildlife from their mesh. Killing the creature is last resort, not first thought.
But there was NO suggestion from the QC that the fox was injured (which would be a defence to putting an animal out of its misery). Instead he made a flippant (and incorrect) comment about the RSPCA not being available where he was in Central London on Boxing Day.
Didn't see the comment about the baseball bat but don't think he's breaking the law there. The law prohibits possession of offensive weapons "in any public place". Not in the home. The trouble would only come if he used it against an intruder and the authorities were not convinced he used it in reasonable self defence.
Sorry, think this turned into an essay!