The Hollow Metastacia, who was responsible for the deaths of Kaien Shiba and his wife Miyako. His ability allowed him to possess his victims; he first did this to Miyako, and made her murder several of her subordinates before killing her. When tracked down and confronted later by Kaien, he said that he regretted "not eating the rest of that female soul reaper". Kaien jumps into battle with Metastacia, but ends up getting possessed as well. Metastacia sadistically taunts a petrified Rukia about her love for Kaien before attacking her. His self-inflicted death is very satisfying, despite him ending Kaien's life at the same time.
The Hollow Shrieker was shown to have been a Complete Monster even before he turned into a Hollow. A serial killer who stabbed a woman named Shibata to death and got off on it, his greatest regret was being killed before being able to do the same to the woman's young son, Yuichi. After he became a Hollow, he sealed poor Yuichi's soul into the body of a parakeet and lied to him, saying that if he could escape from him for three months, he'd revive his mother - and Shrieker would kill and eat the souls of anyone trying to help the child. He gets a very appropriate comeuppance, however - after Ichigo kills him, he is sent to Hell by Rukia for the serious crimes he committed whilst still alive, because the Zanpakuto can only cleanse Hollows of sins committed whilst being Always Chaotic Evil. Shrieker is the only Hollow seen to receive this treatment, which says a lot about how bad he was - even Grand Fisher, listed below, was only a Complete Monster whilst a Hollow (or otherwise hadn't committed sufficiently serious sins whilst human).
These are the Bishounen Line for the Hollows. At best, they're likely to be a Jerkass. At worst.... well, keep reading.
Szayel Aporro Granz, the 8th Espada. As a Mad Scientist for the arrancar, Szayel likes nothing more than pulling the organs out of people via voodoo dolls and then smashing them to bits. In particular, before the organ-breaking began, he toyed with his enemies' heads by ripping apart one out of two voodoo dolls in half, only to reveal that the only parts that can be felt from a voodoo doll are the toy-like organs on the inside. Beforehand, he had specifically modified his underlings to serve as healing aids - by devouring them whole, he'd be able to heal himself. Now, in regards of what he did to poor Nemu...well, bluntly put, one of his abilities allowed him to invade any convenient host (the petite Nemu) that had maintained physical contact with him during his death, devour Nemu from the inside to reconstruct his original body, horribly enlarging Nemu's stomach and emaciating her to near-death, and then burst fully grown from her mouth head-first, leaving Nemu an anorexic husk, whose condition was so bad that she had to get groped by her father to get healed back to normal. What's worse is that the amount of time it took for that to finish was just a tad bit too long, giving the impression that he was initially trying to burst through her "special opening", before deciding to move out of her mouth instead. He also deconstructs the trope of Avenging the Villain by stating that he is the younger brother of Ilfort, an arrancar that Renji had killed, but has no intention of avenging him. In fact, he infected his brother with mini-spy cameras in order to gain an advantage in their fight, even comparing the loss of his brother with the loss of a box. All in all, he is so terrible that he is one of two of the Espada to end up in Hell (the second being Aaroniero Arruruerie).
After using Ashisogi Jizo on Uryu to take away his ability to move, he says that killing him would be just as easy as strangling a baby. Based on what you know about him by this point, you probably can wonder if he's talking from experience.
The anime cuts out a certain part of the manga where he actually explain what experiments on the Quincy he performed: pure acts of savagery and sadism with zero scientific value, unless you are the most fucked up scientist who ever lived. He's the Josef Mengele of the Soul Society. As you read the following quote, note the expression on his face while he says that. One of the most insane Nightmare Faces around.
I said it in the beginning...I've finished studying the Quincy. Finished studying! Observed the result of all kind of physical and mental stimuli! Opened skull alive... Forced one to burn his own son to death. Chopped up bodies. Ground bodies. Studied them until they were rendered piles of lifeless limbs!
Longtime Big Bad Sosuke Aizen has pretty much qualified for this since he was first outed as a villain: he manipulated his naive lieutenant Momo Hinamori to be devoutly loyal and supportive of him by adopting a kind professor's facade, and coldly stabbed her while hugging her, nearly shattering her sanity and incapacitating her for well over a month. The kicker? His manipulations were so successful that she still believed he was innocent, even after nearly dying by his hand.
Continuing further, several years beforehand, Aizen had forcibly spawned monstrous abominations inside eight shinigami officers to test out his latest experiment, coldly slashed or insulted the few that actually managed to maintain any sort of sanity to accelerate the process, intended to murder every last one of them almost immediately thereafter, and when that failed, deviously placed the blame on several of his superiors instead, thereby prompting the feudal government to (attempt to) exile or incarcerate each of them. What's worse is that, by transforming the eight officers into Hollow hybrids, Aizen had destroyed the officers' chances of ever being accepted back by their friends and colleagues, having been turned into the Soul Society's most feared and detested of adversaries, as well as sullying the reputations of the three exiles, all three of whom were assumed to have been dead, disappeared or were painted as co-conspirators by their friends, colleagues and superiors.
Aizen wanted to negate Yamamoto's Zanpakuto, so he turned Wonderweiss Margera into an zanpakuto-negating arrancar. In essence, Aizen erased all of Wonderweiss' intelligence, speech, memory, reasoning and emotions, in order for the ability-negating aspect to work efficiently, all so that Wonderweiss could negate the abilities of one person. It's so bad that Yamamoto himself, a hardened veteran of countless scars and battles - is disgusted to the point of actually calling him out on it, Aizen justifies it by claiming that, since hollows (Wonderweiss' species) lack a purpose in life, turning one of them into an autistic doll was his way of giving Wonderweiss' existence meaning, retorting that Yamamoto was the cruel one for killing Wonderweiss and destroying the very same purpose that Aizen essentially forced upon Wonderweiss. Not done yet: Aizen modified Wonderweiss to negate Yamamoto's fire-generating abilities, but made it so that those very same flames were sealed inside of Wonderweiss, who promptly exploded, nearly destroying an entire city almost immediately after his death. Then there's the implications of Wonderweiss not being that willing to do this in the first place.
Of course, Aizen managed to beat the rap by being a Magnificent Bastard for the majority of the time.... which makes his act of tricking Hitsugaya with an illusion into stabbing his childhood friend, Hinamori - the very same lieutenant who Aizen broke beforehand all the more horrific, and finally cemented him as an asshole through-and-through by much of the fandom, even several of his own fans. It's really clear here that Aizen could have effortlessly created the illusion they were attacking out of thin air. He actively chose to use another person for it, and at that specifically chose Hinamori whom he had already messed with.
Also, once he became a Physical God, his very presencewas so powerful that it caused anyone who was near him - many human civilians - to blow up, killing over a dozen civilians, while he casually walked his way through a city. Why, you ask? Well, to give you an idea, when he met a human that was spiritually-aware enough to actually see souls, Aizen warned the guy not to get near to him....just for Aizen himself to walk closer to the human in question, promptly killing the guy seconds later; it was all for him to show off his new-found powers, as a means to quench his narcissism. After that, when he finally met Ichigo's human friends, he planned to horribly butcher them in the same manner as he did to the previous human from above, then hang up their corpses on display just to get a reaction from Ichigo.
The bastard even lampshades his Complete Monster status - he claims to be above such "petty" thingsas ethics and morality, as he claims they would serve no other purpose than to limit his potential. In fact, this is why Aizen is such a terrifying and despicable person: he sees himself as an Übermensch who is above everyone else, and is thus exempt from the notions of morality and decency adopted by "lesser beings", even mocking them as useless notions only the weak would cling to. Therefore, in his mind, he has the right to do whatever he wants, since he's so powerful that nobody can stop him from doing so.
Kouga Kuchiki stabbed straight through his most loyal servants' chest, because he thought Muramasa (who had just released him from imprisonment) betrayed him earlier and allowed him to get imprisoned. After finding out that Muramasa didn't abandon him, Kouga breaks the sword connecting his soul to Muramasa, forcing Muramasa to undergo a hideous transformation into a One-Winged Angel. Griffithwasn't that cold.
Kageroza Inaba, the 7th Seat of the 12th Squad. His behavior towards Nozomi is rather questionable, to the point where her default reaction for Inaba is like that of a rape victim. It turns out he really, REALLY wanted to get close to her to have a Fusion Dance with her so that he could become his original self again, no matter what.
Kokuto from the fourth movie, Hell Chapter, deliberately invokes this attitude to get Ichigo to break the chains keeping him in Hell. All in the span of 30 seconds, he incapacitates Renji, Uryu, Rukia and Yuzu, and taunts Ichigo with the lifeless body of his younger sister. Why? Just to piss him off.
