Shrieker might have just been a Hollow, but he pushed the envelope when it came to evil. A Serial Killer in life, he died shortly after murdering a woman named Shibata, when her infant son Youichi trips him out of a window. Stealing Youichi's soul and putting it inside of a parakeet, the Hollowfied Shrieker promises that if Youichi can escape him for three months at a time, he'll bring the child's mother back to life (something he can't do). Able to track Youichi's soul, Shrieker kills and devours anybody who tries to help Youichi, feeding off the psychological torment it causes, as much as the actual souls. Targetting Chad, Ichigo, and Rukia for attempting to aid Youichi, Shrieker set the heinous bar higher than any Hollow before or since.
These are the Bishounen Line for the Hollows. At best, they're likely to be a Jerk Ass. At worst.... well, keep reading.
Szayel Aporro Granz is the Eighth Espada and the resident Mad Scientist among the Arrancar. A sadist and a player of games, Szayel creates voodoo dolls out of his victims, then proceeds to torture them to death, rupturing one organ at a time, as he cheerfully does to Renji Abarai and Uryu Ishida. He's also the worst boss of any of the Espada, modifying the bodies of his subordinates so that in a pinch he can convert them to raw energy and devour them with ease, using their lives to fuel his own. When his battle against Mayuri Kuortsuchi goes bad, Szayel uses his rebirthing technique to possess Kurotsuchi's daughter Nemu, infiltrating her body, and draining her of her life's energy, before bursting forth, reborn. A sickening Evilutionary Biologist, who regards himself as the ultimate lifeform, Szayel takes pride in his unique brand of depravity, and regards everyone, including his late brother Ilforte Granz, as beneath him, and, ultimately, disposable.
Mayuri Kurotsuchi, the Gotei 13's resident Mad Scientist who frequently conducts experiments on living subjects. When he's first introduced he used his unwilling subordinates as human bombs against Ishida, and, when one of them survived, Mayuri blew him up anyway because bombs "aren't supposed to come back." During his fight, Mayuri reveals to Ishida about how he conducted horrible "experiments" upon his fellow Quincies, among which included opening their skulls while they were still alive, chopping them and grinding them up, and even forcing one to burn his son to death For Science! One of his victims was Ishida's beloved grandfather whose mutilated corpse Mayuri keeps a picture of on his person, even proudly showing it off to Ishida during their fight. After the Soul Society Arc Mayuri no longer acts as an antagonist to the heroes, but he's still the most vile and depraved out of all the Gotei 13, and it's clear that he only works for the good guys for the chance to work on his sadistic experiments without repercussions. In fact, Mayuri was willing to kill all of his fellow Gotei 13 members by destroying the pocket dimension they were in just to take out Aizen. He was only stopped from doing this by his subordinates' interference. Mayuri also displays a callous disregard for the life of his "daughter" Nemu. He savagely beat her after she had been severely injured and, when an enemy had reduced her to a withered husk, Mayuri's only response was a spike of scientific fascination.
Sosuke Aizen is a sociopathic ‹bermensch with a god complex who considers himself above the notions of morality and decency adopted by "lesser beings." Uncaring for anyone but himself, Aizen has a habit of using and betraying anyone close to him. Examples of this include testing out an experiment by turning his former captain, Shinji Hirako, into a monstrous abomination along with seven other shinigami officers, attempting to murder his subordinate Harribel once she had outlived her usefulness, and impaling his devoutly loyal lieutenant, Momo Hinamori, while hugging her, leaving her traumatized for months. Later, Aizen would outdo himself by creating illusions which tricked Momo's best friend, Hitsugaya, into nearly killing her. Aizen also used Wonderweiss Margera as a tool, erasing all of his intelligence, speech, memory, reasoning and emotions just to turn him into a Power Nullifier against one man's weapon. Once Wonderweiss had successfully absorbed the flames of this person's weapon, Aizen had arranged it so that Wonderweiss self-destructed, nearly taking out an entire city with him in the process. Finally, when Aizen had achieved physical godhood, he tested out his abilities by calmly walking through a human city, knowing that his powers caused anyone who came into close proximity with him to blow up. In the end Aizen ended up killing over a dozen of civilians, and even planned on doing the same to Ichigo's human friends just so he could mock Ichigo by hanging up their corpses on display.