Its a hypothetical anyway, but there is a vast difference between murder and a zap from an electric fence. Quite rightly, murder is categorised as unreasonable force (the vast majority of burglaries are unarmed - at least, when I last checked these stats), unless murder is in the intentions of the burglar. By the rationale that suggests this to be the correct approach, I would be out shooting kids on the allotment (I hope we're all more civilised that that!). I would be surprised if case law has uncovered any examples of prosecution relating to an ordinary farm-style electric fence, but if you could show me otherwise, I'd be interested.
Nevertheless, a sign on the fence wouldn't be a problem - cats can't read, so far as I know (then, dependent on age, neither can children).
An electric fence that gets a small bird is incorrectly wired - it shouldn't earth unless you're touching the ground. But, if there are other means which work to deter cats where they need deterrance, it probably is a case of overengineering to bother in the first place.
cheers,
Derek