Castle TPKloft
There’s a fine line between playing a monster smart and being a dick. You want boss monsters to be deadly and scary, a challenge to your party, but if you play a monster with the cunning and underhanded tricks of a player it quickly becomes frustrating for the players, as if the DM is cheating. Dirty tricks are very one-sided in the DM/player relationship.
This very much results from the authority a Gamemaster already possesses and the absolute necessity of trust in the relationship. You have to trust that a gamemaster is not cheating and (unfairly) changing monster statblocks, hit points, or die rolls – aka “fudging”. The GM already wields a ridiculous amount of unchecked power, so if a monster is being cheap or dickish it’s can border on an abuse of trust.
This also intersects with the narrative tropes of the genre. The Evil Overlord List is a thing for a reason. The Big Bad Evil guy should just crush the level 1 heroes, or kill would be heroes in their sleep, or employ giant death golems with innocent villagers and kittens strapped to the construct. Especially since the DM knows the player character’s weaknesses and plans, and a smart villain could (should?) be very effective at slaughtering a group of heroes. If played straight, a villain like Strahd should make an example of a group of adventurers at level 6 – when they first attract his attention and before they’re even remotely a threat – and not wait until they’re almost powerful enough to face him one-on-one.
I’ve been DMing several campaigns, and one of my favourite tricks was to avoid crushing the PCs with the overpowered evil lord. Instead, I made the villain recruit them through some mind-shielded minion to attack some good-aligned enemy of said villain, producing false evidence of the evilness of such an opponent. This way the PCs could be disposed of by someone else, weakening another enemy in the conflict, or even better they would become outlaws and be perceived by the common folk as evil minions themselves.
I think that the true strenght of an evil overlord is his/her ability to plot such a downfall and be delighted, rather than just an exhibit of brawns and magical power.
But as a DM I also did cheat several times, always with the goal of having my players enjoy the game in the best possible way. Sometimes letting them succeed in a narrow escape, sometimes having a hated foe fleeing for a later meeting, or lasting through a serie of lucky hits by the PCs to make a battle truly memorable.