Horrible bosses are some of the most formidable opponents for hardworking anime characters. The worst examples of this archetype might have issues with poor management or inefficient delegation skills. Tamanawa from Oregairu is a prime example of this type of boss. On the opposite side of the spectrum, overseers are executing their employees, like Frieza from Dragon Ball Z, who vaporizes his underlings when they displease him.
The term "boss" is rather broad, since it includes people in a variety of supervisory positions. One terrible boss, Ton from Aggretsuko, works in an office setting, while Akira Midousuji of Yowamushi Pedal makes life hell for his high school bike team.
If you work for a villain, your greatest concern might be getting beaten up by the good guys - or it could be whether your boss will kill you for looking at him funny. Frieza vaporizes his henchmen with little to no provocation. If a subordinate says something to upset him, he'll hurl them out of a spaceship, killing both the offending party and anyone else present. If they fail to fulfill his directives, he'll kill them without hesitation.
Through most of Assassination Classroom, Gakuhō Asano seems like a bigger villain than the tentacle monster threatening to blow up the planet. Asano, the principal of Kunugigaoka Junior High School, sets up a class for the school's lowest achievers on the outskirts of campus. Not only does he treat these students like they're inherently inferior, he actively discourages them from improving their scores. Why? Because Asano believes having someone to look down on will inspire other students to succeed.
This is terrible for the students - and for his employees. A teacher's job is to help their students succeed academically, which is impossible if their boss is trying to prevent it.
Bosses who prioritize their own goals over the physical safety of their employees are already bad news, but bosses who go out of their way to ensure their employees' demise are the worst. Mayuri Kurotsuchi doesn't necessarily like killing his employees - he doesn't care if they die in the name of science. Rather than seeing his underlings as humans, he sees them as fodder for his experiments.
This leads to Kurotsuchi turning his subordinates into bombs (without getting permission or even telling them ahead of time) to help him apprehend Uryū Ishida and Orihime Inoue, whom he wanted for his unethical experimentation.
Ton is a terrible boss who intimidates his employees even when he's aiming for compassion (which doesn't happen often). He goes out of his way to make Retsuko's professional life harder, passing excessive work to her and then ridiculing her when she can't meet his absurd expectations. He insults his female employees by outright stating their subpar work is due to their gender, and assuming they'll all run out the door the instant they get married.
He also drunkenly insults Retsuko via rap battle, which is a big no-no. While Ton isn't killing his employees like some other terrible bosses, it's extremely taxing to work for him.