Always consider alignment as a tool, not a straitjacket that restricts the character. — Oft-forgotten sentence from Advanced Dungeons & Dragons, 2nd Edition Player's Handbook
A character who does things that don't make any sense, just because it's a "good" or "evil" thing to do. It's not that they've lost their goal — they're actually true to their good or evil alignment, but at the cost of every survival mechanism developed by humanity.
This goes way beyond being a Slave to PR or Card-Carrying Villain. Nor are these characters always Knight Templars. A Knight Templar is not necessarily Lawful Stupid, although they are always Lawful Neutral or Lawful Evil - with fanatical devotion to the word of law and absolutely no concern for the spirit.
For more on Lawful Stupid and Chaotic Stupid, check out the LJ post by The Ferrett in which he introduced the terms. However, note that his definition of Chaotic Stupid is closer to our usage of Stupid Evil.
Also compare Dudley Do-Right Stops to Help (good) and Dick Dastardly Stops to Cheat (evil) for when pandering to MORAL alignment actually hinders the objective.
Alignments covered as sub-tropes include:
Note: this page is currently undergoing editing to split it into separate pages. Please put new examples on each alignment's page, and not here.
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Examples of Chaotic Stupid/Stupid Evil
The old World of Darkness games had at least one faction in every playable race that was frequently chosen by players of Stupid Evil or Chaotic Stupid characters. These groups had a tendency towards extreme violence, madness, and generally being Darker and Edgier than the other factions of that race. Whether or not these factions were inherently Stupid Evil or Chaotic Stupid is debatable, depending on how awesome you believe a given faction to be.
Vampire: The Masquerade: Any member of the Sabbat for Stupid Evil, especially the faux-transhumanist Tzimisce. The Malkavian Clan was pretty much only ever played as Chaotic Stupid, as players went out of their way to play "wacky" insane people. One of the most notorious kinds of Chaotic Stupid Malkavians is known as the 'fishmalk', played by the kind of people who think "crippling insanity" means "hilariouslyrandom", with one of the inspirations for the nickname being an illustration where a Malkavian is fondling and making kissy-faces at a fish.
Werewolf: The Apocalypse: The Black Spiral Dancers, and Werewolf-related entities like the Fomori. In-setting, the evil corporation Pentex frequently engaged in Stupid Evil activities through the bad behavior of its subsidiaries.
Mage: The Ascension: The Nephandi for Stupid Evil, Marauders for Chaotic Stupid. Curiously, the more popular choice over both was the Lawful Stupid Technocracy, most likely due to a possible Alternate Character Interpretation in which they are protecting humanity and rationality from the actions of destructive Tradition mages while clearing their own ranks of corruption.
Changeling The Dreaming: Unseelie fae got some Character Development as the Changeling The Dreaming product line expanded, but many players did not get the memo. Initially, the Unseelie were portrayed as rather jerk-assish. Poorly played Unseelie Pooka were worse than the most annoying of fish Malks, though at least Changeling allowed for levity and it wouldn't always clash with the game's mood.
Hunter The Reckoning: Any of the Creeds - philosophies towards monster hunting and their associated abilities - could potentially fall into one of the Stupids. Common examples include Stupid Evil Avengers and Waywards, Stupid Good Innocents and Redeemers, Lawful Stupid Judges, and Stupid Neutral Visionaries and Hermits.
Demon The Fallen: The Raveners, a faction of demons dedicated to destroying the earth out of spite, became a home to many of the game's Stupid Evil characters.
Game Masters for Dungeons & Dragons and Pathfinder, if they have more than two nights of experience, are well aware that whenever a player writes down "Chaotic Neutral" for their alignment, the odds are strongly in favor of either 1) chaotic stupid or 2) stupid evil, and trying to pull a fast one by not honestly saying they plan to play an evil character. Yes, some players can play a CN character without being little more than a headache to the GM and other players, but many more cannot.
The second edition setting Planescape had several organizations with distinct, powerful philosophies. Good players found lots of interesting conflict and choice in them. Bad players had the doors to stupid-evil and chaotic-stupid thrown wide open for them. There were the Mercykillers, who cruelly punished those guilty of crimes, the Xaositects, who worshiped chaos, the Transcendent Order, who tried to purify action without thought, the Bleak Cabal, who were nihilists, the Dustmen, who believed everyone was already dead and needed to purge themselves to escape reincarnation, and the Fated, who believed charity was bad and individuals should earn or take what they want. Stupid evil and chaotic stupid characters could justify anything with these.
In the original campaign for Neverwinter Nights 2, you find a sorceress named Qara as a companion. She's Chaotic Neutral according to her alignment, but she could fool you. All of her actions, every single one of her words and absolutely any dialog options you have with her that makes her like you better are outright Chaotic Stupid AND Stupid Evil. If you're True Neutral you'll learn to dread your conversations with her very quickly, as all options always give a bunch of points towards any of the extremes of alignment, also giving or taking a bunch of influence with her. With no middle terms.
Which is quite odd, considering that Sand, a wizard you can get to follow you and is the exact opposite of Qara for most things, is a much better character. He is Lawful Neutral, but understands the law is not absolute. He is a jerk most of the time, but devotes himself to your trial in order to take a few punches at bigger jerks. His opinion is that it is better to be a coward and live than to be brave (or stupid, in his opinion) and die. Horribly, most likely. He only follows you afterward to potential danger because he is already caught in the mess.
In the Mask of the Betrayer expansion, the system is a bit better. You find more instances where being evil is not being brain-dead and it is a lot easier to keep a True Neutral alignment without looking like a turncoat psychotic. Even though the Spirit-Eater your character turns out to be is still a pain to the alignment (some powers give you points towards one way or another, without considering that you may simply want to use them for the convenience, if you're neutral), it was later fixed with patches. Not to mention that you receive much more chances to affect the influence of followers, so you can screw a bit during conversations without screwing the relationship itself.
Mortal Kombat does this in Mortal Kombat Deception, in light of shifting the dichotomy in-game towards a conflict of Order Versus Chaos. You have characters that side with one of the fiction's two evil Big Bads simply because they perceive one as distributors of order or chaos, and the other as the opposite. Both the lawful Big Bad and the chaotic Big Bad will inevitably cause total destruction and unmaking of the realms, either instantly and controlled, or wildly via turmoil. Forget that you'll have your life probably destroyed in the process, because victory for either evil of the Big Bads will obviously inevitably cause evil, the characters go along with it because they're militantly obsessed with order or chaos as abstract concepts only. The characters are stupid in the sense that they contradict with the ideals of justice or safety associated with order, or self survival associated with chaos, rather than using the ethical alignments for their own morally neutral goals. While we don't necessarily see every example, they tend to bounce between good and evil, just to see their abstract values carried out in any way.