Complete Monster/Mario

""Yes, it was I who broke the Dark Moon! Not a difficult task when you have a power-enhancing jewel on your crown like this one. And now that all the ghosts of Evershade Valley are under my control, it's time for the rest of you to become my subjects too! You, that annoying scientist, and all your friends will look great hanging on the walls of my throne room! Aaaaaah ha ha ha!" -King Boo revealing his plan in Luigi's Mansion: Dark Moon."

While a very light-hearted, fun loving kind of series that is known for it's happy and cheery aesthetics, even Mario isn't immune to these guys showing up every now and then. Needless to say, Mario's not going to party or race with these folks.


 * The third game in the Paper Mario series gives us a rather vile example in Dimentio, a sadistic sociopath who puts the Monster in Monster Clown. While he's introduced by forcing a benevolent ancient guardian to fight Mario to the death, he mostly seems like a goofy, simile-obsessed minion of Count Bleck's aside from that one nasty moment when he uses his own comrade, O'Chunks, as a guinea pig for his floral sprout project. It isn't until near the end of the game however when his sociopathic nature becomes far more apparent, starting when he sends Mario, Peach, and Bowser to the Underworld after he did the same to his ally Mr. L (A brainwashed Luigi) while casually cracking jokes at their expense (Don't worry, they get better). And at the game's climax, As you can tell by now, he's pretty much the Nintendo equivalent to Kefka Palazzo.
 * While King Boo tends to be someone willing to race or play sports with Mario and friends in spinoffs or shows up as a boss to fill in space in other games, it's the Luigi's Mansion games where we see the guy at his worst. He started off as a Well-Intentioned Extremist in the first game who was very fond of his Boos and went out of his way to rescue a powerful one named Boolossus from one of Professor E. Gadd's paintings... but after being defeated by Luigi, he completely and utterly casts away any and all redeeming qualities in the sequel. Thirsty for revenge against his hated enemy, King Boo shatters the Dark Moon of Evershade Valley and turns the peaceful spirits living there feral and nasty, enslaving them against their will. After sending them out to cause mayhem in the valley, he kidnaps Mario and a couple of Toads and imprisons them in paintings, while they're completely aware and unable to move. And in case if you thought that he still cared about his fellow ghosts, that's a big, resounding NOPE. King Boo performs all sorts of nasty experiments on his ghost minions and happily treats his Boos as disposable pawns, throwing them at Luigi with reckless abandon. In Luigi's Mansion 3, he is set free by Hellen Gravely, and together they gather Luigi and his friends in the Last Resort hotel in another attempt to trap them in paintings. When Hellen and her staff are all captured, King Boo dismisses them as worthless failures and eventually tries to absorb the entire hotel, including all the captured ghosts and his own Boos, into a group painting just to capture Luigi. Since most of the series' darker villains are relegated to the RPG's, it's almost shocking to see this sociopathic madman in one of the more mainstream titles where the villains tend to be Bowser and his goofy henchmen. It should be noted though that King Boo only counts as this in the Luigi's Mansion series, particularly since the second game: he doesn't qualify in any other games that feature appearances by him due to being willing to participate in parties and sports with Mario in some or being a generic villain in others. Thanks to Negative Continuity being in effect, he only meets this trope's qualifications in the Luigi's Mansion series since Dark Moon.