During Volume 7, Albedo makes an offhand comment about how the guild "Ainz Ooal Gown" means nothing to her and she has even made a new flag in honor of Ainz. Naturally, this shocked a lot of readers into thinking why one of (if not the) most loyal follower of Nazarick would say and do such things, but when you think back to what Ainz originally did to her settings, it all makes sense. Ainz programmed her to be in love with Momonga. Let me repeat that, Ainz programmed her to be in love with Momonga, not Ainz Ooal Gown. She's only doing what she was programmed to do.
Albedo's decision to kill the 41 Supreme Beings (except for Momonga of course). Now of course if you're just reading it for the first time, it's natural to think that this is absolutely ridiculous and that Albedo is truly an ungrateful cunt, but if you really think about it, can you really blame her? Imagine you were created to be an NPC by a bunch of players and they can customize you to be whatever they want. And there's nothing you can do about it. They can change and alter your personality, powers, and mannerisms to whatever degree they choose, making you nothing but an obedient slave.
Not only that, but also the people you were created to protect and swore your loyalty to eventually forget about you completely and abandon you (in truth, just not play the game anymore). It was even stated that 37 of the original 41 members of the guild quit and the last four remaining (save for Momonga) were barely active. Hell, if it wasn't for Momonga contacting his last remaining friends in the guild to show up for the final day before the game closes, they wouldn't have even bothered to show up. Though the Guardians apparently show no resentment towards the Supreme Beings, one can only wonder how they truly feel on the inside.
Though it is true Albedo's reasons for killing the other members might be what was posted above, her reaction in Volume 3 when watching Ainz become depressed when he sees his old guild mates's equipment and realizes how much pain they're causing him could also be a major factor.
Alternately, the NPCs are shown to be loyal to their creator first, Nazarick and the guild of Ainz Ooal Gown second. Momonga's very last action in the game was to edit Albedo's settings. In addition to his edit of making her fall in love with Momonga, her in-game edit history might retain the memory of that editting, meaning she also views Momonga as her creator.
According to Japanese reading viewers of the anime, the reason that Albedo had questionable loyalties is that her setting has her as a fallen angel, corrupted by the guild, Ainz Ooal Gown, and that she has a personality like... well, several famous Asian empresses who ensured their power through bloody, horrific methods, but comparing her to Bloody Mary wouldn't be harsh enough so there's no real western counterpart. Being in love with "Momonga" just means that instead of setting herself or her child in a position of absolute authority, she wants Momonga to have that position... the fact that he's calling himself something else is no doubt a thorn in her side, though.
It might seem strange at first glance that Albedo seems completely unperturbed when Ainz admits that her love for him was artificial, that it was something that her original creator had not intended to be part of her personality. However the denizens of Nazarick see the Supreme Beings as gods and Ainz as the head of the pantheon. They exist only to fulfill the wishes of the Supreme Beings. If the Guildmaster of the Supreme Beings decides to make you love him, that is not Mind Rape but is instead an honor bestowed upon you.
During Operation Gehenna in Volume 6 of the Light Novels, Evileye and Nabe had to hold the line against the minions of the evil archdevil Jaldabaoth. Nabe fights three of them while Evileye takes on Alpha and Delta. The matchups make a lot of sense since Yuri and Shizu are the two Good aligned Pleiades who are unlikely to lose their temper and accidentally kill Evileye.
Why was Narberal of all the Pleiades chosen as Momon's partner? Simple, the need to keep party balance and several disqualifying factors. Yuri is the best fit for the sake of personality but she is also a melee combatant. People might accept that a "man" with Momon's stature could be extremely strong but Yuri? Not so much. Not to mention that most of the parties we see are balanced between might and magic, so a party consisting of two melee combatants would definitely feel very out of place. Besides, Yuri is the head of the Pleiades and needs to be present to coordinate and control her sisters. Lupusregina blends in the best with humans but as a faith-based caster that does not follow any of the existing religions, her magic will mark her as an outsider. In the unlikely event that Momon is injured, she can't heal him either as healing magic hurts undead. So a cleric that is never seen healing her partner is somewhat suspicious. Shizu and her guns are right out, they don't even exist in the New World. Solution too can have a hard time keeping up the masquerade in a setting where she is likely to be attacked. It is unknown how well she can simulate the injuries that a human body should suffer when attacked but it takes just one slip up to reveal that she is not human. Entoma is a cheerful man-eating abomination whose powers will definitely invoke horror in everyone who witness it. Therefore Narberal, despite her quirks, was the only viable option.
Even if we ignore gender or the Pleiades, the other denizens of Nazarick have similar issues. Most can't even pass for human, or demi-human, and of those, Albedo and Shalltear have to be ruled out because taking one inevitably causes problems with the other, and that's not even taking into account their respective personality issues. Sebas Tian would be viable as a Bare-Fisted Monk except bringing a butler to an adventuring lifestyle would be highly suspicious. Aura would be viable, and valuable, as a ranger with her bow providing ranged attacks, but she and Mare tend to come as a matched set, and Mare's druid magic also includes healing spells, so that rules him out for the same reasons as Lupusregina, and lastly even if that wasn't an issue, Momon would still have problems with the local Fantastic Racism towards dark elves, especially in the unlikely event he had business with the Slane Theocracy which is also a client of the Adventurer's Guild. Lastly, all of those listed occupy high positions within Nazarick as Floor Guardians and the like, and are more often than not essential to the day-to-day functioning of the Tomb. They cannot be removed from it for extended time periods.
Ainz plans to make the Sorcerous Kingdom of Ainz Ooal Gown into an egalitarian utopia where anyone, regardless of race, can live in peace. Demi-humans, heteromorphic races and humans are expected to live in harmony and prosperity under his reign. It is nothing more or less than the guild of Ainz Ooal Gown in a larger scale, the only place that Ainz felt happy and accepted.
Ainz's original character name, Momonga, may seem silly once one knows it is from a species of squirrel, but think of the setting in which Ainz had made his character, Yggdrasil. In Norse mythology, it is said that Ratatoskr, a squirrel deity, traversed the world tree back and forth to spread information and gossip and watch the gods duke it out. Ainz himself has, since coming to the New World, been looking to gather information and, like the squirrel god, use it to his own advantage. He may have even originally named his character in a subtle nod to Ratatoskr.
The Ainz Ooal Gown guild had two requirements for joining: play a non-human character and being employed in real life. Given how often they used cash shop items, they'd have to be able to pay for them.
Entoma, despite being ostensibly an insectoid herself, is The Dreaded to other insectoid creatures in Nazarick and even unsettles the vastly more powerful Cocytus. She is not a normal insect but a spider, one of the apex predators of the anthropoid world.
When Evileye first meets Momon, the texts specifically calls out that she gets excited "down there" and she note's that this is the first time she's felt like a woman ever since she was turned. She attributes it to him arriving and acting out the dashing hero role, but there is some evidence pointing in an entirely other direction. When Ainz had first shown his "Phenomenal Cosmic Power" to the floor guardians Shalltear had a very similar reaction, though it was much more pronounced. Possibly due to the difference in intensity, it could very well be that Evileye got all hot and bothered because she was passively sensing a high tier undead. In traditional roleplaying games, whenever a living creature was exposed to a large amount of positive energy, they were filled with thoughts of having children or something similar as positive energy is 'life energy'; with the undead it's the same effect (somewhat) with negative energy, usually depending on the type of undead. As Vampires are one of the few undead capable of reproduction, where else would Dhampir's come from, having a similar arousing effect could be expected. And as Ainz is a powerful undead and most likely gives off a negative energy aura, it seems fairly plausible that Evileye caught a whiff of something.
Why would Princess Renner so quickly and easily betray her country and pledge fealty to Nazarick? Well simple, her emotional ties to her Kingdom are nil. The people around her have always treated her poorly, she can see her beloved father suffer at the hands of his factitious nobles and their classist society looks down on the man she loves due to his low birth. Her odds of claiming the throne are also laughable, if her brother dies it is far more likely that a powerful noble will claim the throne through marrying her than Renner ruling in her own right. Moreover they can offer something Renner could never get otherwise. A legitimate marriage to Climb, presumably as the vassal monarch of the Kingdom.
The denizens of Nazarick speak of Ainz having an heir through either Shalltear or Albedo as if women, one of whom is also undead, could bear a lich children. However from their perspective the Supreme Beings created them out of nothing, giving them life where there was none. Surely impregnating a woman would not be beyond the capabilities already demonstrated by people who made children out of whole cloth.
The Jaldabaoth confronted by the Holy Kingdom Liberation army after his initial appearance in their capital is not Demiurge but different summoned Demon Lords of Wrath. The deceit works since few living people have ever even seen Jaldabaoth and lived to tell the tale (roughly four people, three of which are resident in the Kingdom) aside from the citizens of Nazarick. This also plays into the notion that many demons have a One-Winged Angel form and allows Ainz to conclusively kill "Jaldabaoth" without any further need for chicanery.
Hamsuke's supposed and much lauded wisdom is remarked upon by everyone who meets him. As an animal, by D&D 3.5e standards (and Overlord is HEAVILY based on D&D 3.5e) she would have above average wisdom. If she's also druidic (going by her use of runes), then that could bump her Wisdom up even higher, and since she's on par with Gazef, who is considered to be near the 'limit' of humanity (translating to roughly level 5-6 in D&D terms), she'd probably have quite a bit of Wisdom over your average Joe sitting at level 1 or 2. In other words - they're not commenting on how she acts, they're quite literally commenting on her stats.
When Ainz learns from Solution that Sebas may have gone rogue, he sends in Pandora's Actor disgused as himself, Cocytus, Demiurge, and Victim to check on him. Each person there has a layer to why they would be chosen by him given the context.
Pandora's Actor: Being able copy someone's power up to about 80/90%, he can look like Ainz in the event that is turns out to be true. Plus if Sebas is now a threat, Pandora's Actor can turn into someone else and trick Sebas or any potential threats without revealing Ainz's true power.
Demiurge: Ainz is the only one aware of the Irrational Hatred and the reasons both Demiurge and Sebas hate each other. In the event Sebas betrayed them, Demiurge's presence would be enough to potentially rattle Sebas into being less focused than he should be. He is also extremely intelligent and unnaturally skilled at discerning people's intentions, magically and otherwise. If Sebas attempted to deceive them, Demiurge would read him like an open book.
Cocytus: He's, physically, one of the best Floor Guardian's to have in a melee fight. If Sebas does go rogue and attack, Cocytus is physically the best person to stand up to him given his durability. This is even somewhat seen when he easily catches Sebas' punch without trouble. Although Albedo is technically a better counter for Sebas's build, she is also impulsive and violently protective of Ainz's safety, putting her at risk of becoming hostile towards Sebas even if his transgressions, whatever they may be, do not warrant it. Therefore, she was right out.
Victim: A last resort. If Sebas somehow was a major threat and couldn't be stopped, Victim would be a Heroic Sacrifice to disable him and any other attackers.
The "Horn of the Goblin General" has three unknown conditions that never worked in YGGDRASIL, but can apply to "the new world" to allow its full power to be brought out. The light novel makes a point of saying it's not "the goblin horn" but that it's "the Horn of the Goblin General." This leads a reader to deduce two of those three conditions.
1.) The person blowing the horn must be a goblin. Even though Enri is not a goblin by blood, she is a goblin, by adoption, under goblin law. All her previously summoned troops call her "Ane-ue," or "big sister." We have precedent from Volume 8 in the form of Agu, a hobgoblin who is the chief of his goblin tribe because he was adopted by the previous goblin chief, as a baby, and is officially recognized as a goblin by them.
2.) The person blowing the horn must be a General. Enri had at least 3 levels in "General" before she blew the second horn.
The third condition is still vague, but it's either commanding different species together, or blowing the horn in battle, or having a sufficiently large base of operations, and being the chief of Carne Village certainly qualifies. Another possible condition is "the total amount of enemies/the total combined level of the enemies must be X". Since heteromorphic races were not popular in YGGDRASIL in the first place, the amount of players with both the Goblin race and the General class could likely be counted on one hand to begin with, and on top of that, the Horn being known and dismissed as a low-level noob item meant that no one ever brought it to a raid or battle of sufficient scale or level to activate its full abilities to avoid wasting inventory space on it.
While a lot of the Guardians have cruel and sadistic personalities or moments that make them come across as genuinely scary (especially in the light novel when you get to see the POV of their victims), it's always consistent with their character, except with Mare Bello Fiore, due to the story implying now and then that his shyness is only an act and not a real part of his personality like all of the other Guardians' quirks. He seems to act like that and dress like a girl only because Bukubukuchagama designed him like that, but maybe she never wrote him a proper backstory, so when he came to the New World his personality might have been based more on his race, a karma level of -100 and Bukubukuchagama's personality, which is not a mix that would logically give you a shy elf boy who likes dressing like a girl.
There's also the fact that Mare has already proven himself multiple times of being smarter than he looks (and that he is the stronger guardian after Shalltear). Demiurge himself has a friendship or mentorship with him, and he believes Mare is just using Obfuscating Stupidity to hide his true intelligence.
When he kills an Adventurer in the light novel, the narrator points out that despite being apologetic before attacking him and having a face that shows remorse, his eyes displayed no emotion or care for his victim, as if he was simply following a script.
Princess Renner getting the idea that "love" means chaining Climb up in a dungeon and using him as a sex toy comes completely out of nowhere. True, she's a Yandere, but where did she learn that? Someone, somewhere, had to show her. Was she taken to an Eight Fingers brothel and Forced to Watch? With threats that that would be her fate if she didn't stop being too smart For Her Own Good? Was she made to go through it, firsthand? While someone was telling her that's what "love" means? The more we learn about Renner, the more horrific her childhood appears. This girl needs therapy badly, but There Are No Therapists. To double dip the sheer Fridge Horror, Renner's father is The Good King. Its highly unlikely that he would prove to be a neglectful or abusive parent. Who in the hell then is teaching her such concepts?
This Troper believes that her desire to "chain him up like a dog" is not for a sexual reason, but rather because she adores his devotion more than anything and wishes to see that devotion remain forever. The idea of chaining him up is meant to keep him from changing, dying, or even leaving her. She wishes for him to always remain near her and she sees this as her best chance to get her wish.
There might be potential for both possibilities here, when you think about her upbringing, and the people around her, her mindset makes more sense. Look at her twisted siblings, her maids, the Nobles she has to deal with, and even the Knights, all on a daily basis no less, they are all monsters in their own right. Her dad is neglectful actually, certainly not intentionally abusive, but he has to run an entire kingdom that is slowly but surely succumbing to extreme corruption from within. He has absolutely no time for her in any sense that would matter. Except for her father and Climb, and maybe a few others like Gazef and Brain, she is surrounded and influenced by nothing but total scumbags who would happily enslave, defile, abuse, and murder both her and Climb, and not necessarily in that order either. This is the only way she can protect one of the only males she truly loves, cares about, and fight back whenever she can. Where did she learn these things? The people around her of course.
For Platinum Dragon Lord, Ainz represents his worst-case scenario. There is no "divide and conquer" possible here, what he has on his hands is effectively a magical von Neumann machine who has the complete loyalty and control of all the resources of a top-tier guild. Every day, Ainz can create cannon fodder from corpses that are stronger than the vast majority of New World residents without any true cost to himself. Past a certain point, Nazarick becomes self-sustaining as they have so many troops that Ainz can effortlessly replace any losses. Unlike the Eight Greed Kings, there is no one to betray Ainz. Unlike the Six Gods, his forces are derived almost entirely from Yggdrasil and thus are generally much stronger than anyone else can field. You can almost see him going "now what?" when trying to find a chink in Ainz's armour.
Goes even further in that Ainz is very pragmatic and Genre Savvy which means he's already planning for the day when stronger beings will challenge him. More than that, Ainz knows YGGDRASIL's systems inside and out, which means that Ainz knows more about the items Plantinum Dragon Lord uses than the dragon himself.
It's noted that Momonga is actually not all that strong for a level 100 player character, since he had a build more suited towards roleplaying. He's still powerful enough to steamroll entire armies in the New World. One wonders just how powerful a min-maxed level 100 character like Ulbert or Touch Me would be in the New World, who unlike Shalltear would also be skilled in dueling strategy like Momonga.
It's possible that, due to this being 'real life', spells and techniques are just that- spells and techniques. Even if your stats are min-maxed, then 'real world rules' might kick you in the ass. Role-playing spells actually might be equally as powerful as more 'practical' spells in the New World, so it's possible this is the only place that Momonga is on equal ground to his comrades in effective power level.
Also, it's been noted that while impractical, Ainz's build actually made him the most versatile caster in the whole guild. Sure, casters like Ulbert were more powerful, but Ainz knowing over 700 spells, his mastery of chronomancy (time manipulation magic), and having a unique class with long term advantages (Eclipse), made him strong in his own right too. It's also notable that having maxed-out damage potential is far less useful in a world where nothing can withstand even a tenth of that power. Against anything but a high-level Dragon Lord, bringing Ulbert or Touch Me to the New World would be akin to using a nuclear weapon to destroy an anthill.
In-universe, Emperor Jircniv in Volume 10 has such a moment when one of the Four Knights questions whether Ainz is actually the strongest being in Nazarick. Jircniv immediately tries to shoot down that unsettling notion before he remembers that he is far weaker in terms of combat ability than his Knights even though they are ostensibly his subordinates. That this might be the case with Nazarick (and it is — Ainz is powerful but he's far from being the strongest being in Nazarick) is too horrifying for him to accept. That would mean accepting that there's something in Nazarick more powerful than a lich sorcerer king who can slaughter an entire army with a single spell.
One flashback (when the guardians were still NPCs) shows Shalltear being close enough to two players to hear their conversation, which she remembers in the New World. If the guardians were sentient during the game, Albedo might remember her settings going from "nympho" to "loves Ainz". And while we're at it, what did being a nympho in a game where nothing R-18 is allowed do to her psyche?
A subtle one, but considering just how loyal all NPCs are to Ainz (they see him as a mix between a paternal figure and a God who is loving and all powerful), a lot of Ainz's actions and experiments with lower ranking NPCs are really cruel. We got to see how much some of his actions mess up the Guardians' and Pleiades' psyches (both Cocytus and Narberal were ready to kill themselves when they believed they had failed Ainz, and Cocytus' POV in the novel reads more like a terrified child than a warrior ready to die for his master, and you also have Shalltear's mood after the mind control situation), but at least he always talks them down from feeling bad and/or suicidal. That's not always the case with beings like the Homunculus Maids or other staff, which means that despite his best efforts, a lot of his employees suffer a lot under his leadership.
He once made a Homunculus Maid cook a steak to see if she could learn something new. She failed over and over again, so one can only imagine how much fear and self-loathing from failing her master, repeatedly, filled her mind. Even if Ainz told her not to worry, the casual way he did it (since for him it was just a casual experiment) and his expression being hard to read, she might have believed she committed an unforgivable sin and is just living with all the negative feelings. Basically she is living with the guilt of committing the one unforgivable sin in Nazarick (failing a Supreme One, on a direct order no less) and is only alive because of Ainz's boundless mercy. Everyone around her knows this and likely treats her accordingly.
One has to wonder, given the year in the real world (2138) and the state of the world how Ainz even knows the name Hamtaro when he's thinking of possible names for the giant hamster.
So Ainz wanted to use the worker groups to test Nazarick's defenses. Okay. Mission accomplished. How then does the brutal nature of their deaths serve as a deterrent to others that might have the "bright idea" to come investigate Nazarick themselves, which Ainz rather explicitly stated to Foresight? No survivors were allowed to leave and bear witness, let alone deliver a warning, and the adventurers who were hired to escort the Worker teams didn't see any of it. In fact, since Nazarick's location is now common knowledge, and some riches from Nazarick did make it back to camp, the likelihood that others will come to investigate is high. Especially as there are groups like Zuranon that set up camp in such places so as to bring disaster to nearby villages, cities, and kingdoms.
Later volumes explain it a bit. For starters, the very fact that none of the workers (all of whom were relatively strong by New World standards) make it out alive of Nazarick is proof that it's a very dangerous place. Second, manipulating the Emperor into masterminding the invasion provided a pretext for Aura and Mare's attack on the capital.
There weren't really any riches getting back to camp since "Green Leaf" was killed before they could go back and the fact that the adventurers, as well as Momon, would tell that it was too dangerous would stop most people from trying.