Is it a threat to kill though? Again as with the twitter joke, I am not sure it is. Just because you say 'Will no one rid me of this troublesome priest?' doesn't mean you mean it.
No it wasn't a threat to kill, it was a request to kill, accompanied by financial reward.
The troublesome priest quote is telling, because, of course, that particular "not meant seriously" remark led to the death of Thomas Becket. What if somebody had taken the Viscount at his word and committed murder?