Despite the fact that I'm generally refereeing these, days, I still feel sympathy for the players in situations like the OPs,
There are two important facts for referees to remember
1)you know almost everything about your game and gameworld. The only thing you don't control is the PCs. Messing with PC control is immensely dangerous. What's blindingly obvious to you may not be to them, and that doesn't necessarily mean they aren't paying attention, its that you know far more than them, and to be honest probably care about the setting far more than they do.
2) The players don't know what you know and even what they think they know may be wrong, and you still expect them to carry on despite their imperfect information. Most antagonists will trash talk and exaggerate their own power, so I expect players to be critical of the information they receive from any source and compare it to what they know already for consistency. The more you punish players for their lack of information the more you discourage them from playing the game.
In D&D the crippled beggar and the hellgod impersonating a crippled beggar can look identical and sound identical right up to the reveal. There is something distinctly petty about putting the latter in the path of a rude PC who insults beggars with the intention of the hellgod flipping out and killing him.
i have rarely seen killing PCs change player behavior, theres a much better chance of an out of character discussion making some progress. The players who insult all their enemies probably don't mind making up new characters regularly, in my experience. A bunch of them get bored easily and like changing characters a lot, so killing their PC isn't a punishment and could even be a reward.
If i want to create scary NPCs I foreshadow them a lot, and make the players encounter the results of their actions long before they meet at last. Some villianous NPCs don't pan out, for various reasons, but that's ok, because I can always make more. The ones they feel strong emotion about are the keepers, the hate comes first, then they are worth fleshing out.