Me, @istadris and probably many others who have seen their post have headcanoned that King Boo is Luigi's bitter, toxic ex who is pissed off that he was dumped and wants him back. Last night I came up with a literal timeline of how everything went down from LM1 to LM3 and I want to talk about it.
Lm1: KB was just another villain trying to show superiority, didn't even think Luigi would show up to save his bro and didn't have any traps or plans to defend against him, didn't even think he had to, hence why only the boos and poltergeist were actively aggressive towards Luigi, as it was under KB's command since he didn't want him getting Mario back, who was the actual priced possession at the time. Heck, there weren't even any Toads to save from paintings which shows KB's main goal.
Aftermath: KB is defeated and is shocked that he was defeated by Luigi, the second, shadowed brother of the hero of the Mushroom Kingdom with only a damn vacuum and some tiny elemental spirits he found hidden in the mansion. He wants to be mad but he's more curious and intrigued instead. This then leads him to try and learn more about the green man, heck even getting into Mario Kart, Mario Party and other games like this to try and approach him after hearing that Bowser was invited to these events too (He couldn't get into Smash Bros cause then it'd be too many Mario characters).
Start and end of the relationship: Luigi and KB know each other more personally now and they it ends up with them dating. But, KB is still an egotistical king that wants more power and he finds the purple gem stone that he replaced the red one with and it makes him more insane and aggressive. Luigi is scared and has to break up with him, trying to do so as lightly as possible so KB isn't too hurt emotionally and they can still be friends. Take three guesses on how that went based on LM Dark Moon. KB is furious and in denial, not wanting to let go of Luigi, but when he doesn't succeed he's so enraged that he swears to get revenge for this and keep him forever.
LM Dark Moon: KB learns of the new, docile ghosts that stay calm thanks to the power of the floating crystal called the Dark Moon (which I honestly want to look more into) and shatters it to unleash chaos and gain control over them. By this point, he knew how Luigi was the only one who could actually take on ghosts and boos and that he was more closely associated with EGadd so he was certain that he'd be called to do the job. Also because he kidnapped his brother again. The toads were just there and why not trap them to make it more fun? Afterall, the Toads were proper assholes to Luigi in way too many games so he took this chance to punish them for it. I don't think KB created all these mansions, he just found them and used them as his arenas, letting the ghosts figure out the how each one operated and use it to their advantage, with the boos often keeping them in line to the king's goal at hand. Even so, it was obvious that KB was more focused on doing actual harm to Luigi, with the creatures he created thanks to his new gem, to controlling the ghosts and even cursing the broken pieces of the Dark Moon, to going as far as to open a portal and let so many ghosts in that it nearly lead to the dimension collapsing on itself! All of this in a single night! I also want to point out the part in the scene at Secret Mines, where the boos had the ghosts in those red crystals to make them more powerful, taunting them while they themselves looked scared and trying to free themselves before having all that power forced into them and driving them crazy? That probably gave Luigi PTSD since he went through something similar with a worse outcome. And to top it all off, when he had finally all the pieces and thought he could go back to the lab to prepare to get Mario with probably the help of the ghosts, KB sabotages the camera that pixelated Luigi and went him in a pocket dimension that no one would be able to reach them and he couldn't escape from on his own. Honestly, I loved how they didn't have KB move at all besides him floating and staring at Luigi like that the entire time he mocked him, it made him look way scarier, like the calm anger mixed with menace. KB probably tried to make a deal that he'd let Mario go if he came back to him and ruled the land with him but Luigi refused and the fight started, if he couldn't make Luigi come to him willingly then he'd look lovely in a painting above his throne.
LM3: KB. Has. Lost. It. He is royally pissed and more psychotic than ever. He has become more ruthless and desperate to make Luigi pay before he is captured, knowing he couldn't save anyone, especially his bro and the princess. He was truly lucky he found such a naive ghost to willingly and obediently do anything he asked for and made the hotel probably as luxurious as his own castle as a way to show Luigi that this was the luxury he could have had but won't be able to enjoy now that he controlled ever spec of it. He is set on punishing Luigi in the worse way possible, the bosses using terrifying methods to defeat him, all unique and terrifying. Have you seen the T-Rex one? All ghost bosses had some strategy but that boss fight was immediate with no strategy except brute force and desire to kill. And the hotel is filled with deadly traps, spiked walls that slowly close in on you, locked in a room with deadly gas, axes swinging from the ceiling to slice you, the ceiling being threatened to collapse on you in a slow and torturous way, the list can go on but you get the idea. Luigi is greater danger than he'd ever been and at this point, KB is set on having Luigi alive or dead as long as he's his. He is sick to double-death with him slipping away from him when none of this even needed to happen.
