Which boss was the worst? They were each the worst boss that their respective character could have asked for. Nick had paid his dues for years for the possibility of a promotion, only for Harken to Yank the Dog's Chain and keep him trapped in a position he hated with no possibility of advancement. Kurt took pride in his job and his company, and Bobby's irresponsible leadership threatened the company's future. And Dale, who had a lifelong dream of being a loving and supporting husband, had his goal put in jeopardy by a sexually aggressive boss on top of an undeserved reputation as a sex offender that prevented him from finding another job.
Also, all three bosses represent different kinds of workplace abuse:
Harken is a representation of exploitation: Harken strings along and plays pranks on Nick, only to then admit he plans to keep him in the same job permanently and will make sure he can't advance at any other company. Even in his private life, he isn't much better, treating his wife as a possession and showing zero gratitude to Dale for saving his life.
Bobby represents Nepotism. Unlike Harken, who, at the very least, seems to have some work ethic, Bobby hasn't earned his place in the business community and would not get anywhere if he wasn't the boss' son. Because of this, he has no respect or understanding of things like goodwill, competence, or even the value of money. Hence his willingness to injure innocent people, replace good employees with Brainless Beauties, and waste money on drugs.
Julia represents sexual predation in the workplace. While she pays Dale and keeps him around, she still uses him as a toy.
The second film initially seems to have an Artifact Title as the events begin with Nick, Kurt and Dale now their own bosses, with Bert Hanson and Harken more supervisors who don't have direct authority over them and Julia a threat for reasons beyond her past authority over Dale. However, viewed a certain way, Nick, Kurt and Dale are now the titular "horrible bosses", albeit because they're bad at the job rather than because they're bad people, as their incompetence nearly ruins the lives of their employees.
Out of all the bosses in the first film, only Bobby gets murdered. But being the one who dies makes sense: because he's the worst boss in many senses of the word.
Regarding running a company, Bobby is incredibly destructive. Both Harken and Julia are terrible people, but they are more or less competent at running a company and at least keeping Nick, and Dale employed. Bobby intends to suck at every last bit of the company dry until he's destroyed the livelihoods of Jack's employees.
Bobby fully intends to improperly dispose of waste so he can kill thousands of people in a foreign country and save some money. This alone makes him much worse than Julia and Harken, who have hurt far fewer people.
He also destroys people's lives for stupid and petty reasons, like one woman who was fat and another man who was handicapped.
While Harken's murder of him is still wrong, Bobby's death did prevent the collapse of his father's company.
Jack Pellitt seems like a decent guy at first. But considering how Bobby turned out, what if Jack spoiled and neglected Bobby to the point of turning him into a spiteful cokehead? In the sequel, Rex is a wreck partly because Burt was a horrible parent. What if Jack wasn't all that different as a father? Granted, this doesn't at all excuse Bobby's asshole behavior, but it could explain why he didn't even bother to visit Jack's funeral and his incredibly spiteful attitude toward what his father has built.
Harken deciding Bobby had to go wasn't "merely" on a psychotic whim. Having probably looked through Bobby's texts, Harken realizes Bobby is a sex fiend, and would determine that Bobby is the kind of person who would cheat on another man's wife.
All three original bad bosses are twisted mirrors of the protagonists.
Nick wants to climb the corporate ladder, but he more or less tries to do so with a little extra work and a bit of sucking up. Harken reflects dangerous ruthlessness toward anything, even toward an employee.
Kurt is incredibly promiscuous and somewhat childish, to the point of screwing up the protagonists' plans on several occasions. Still, he doesn't let his need for sex completely consume his morals. Bobby, meanwhile, wants to run everything his father built into the ground to feed his demented lusts.
Dale wanted to be a family man and was unfairly labeled a sex offender. Julia has no genuine respect for anyone's boundaries and is guilty of many instances of rape.
Dale's goal in life is to become a husband and raise a family. His becoming a dental hygienist makes a ton of sense: it is the kind of job that would pay him a decent salary while giving him the time he needs to support a family.
Fridge Horror
As stated, we find out in the sequel that Julia is sexually attracted to 14-year-old boys. Considering that she's not above taking advantage of patients after gassing them, God only knows how many underage patients she's molested in the past.
Nick was planning to kill Pellitt by lacing his cocaine with rat poison before getting cold feet (and before Harken beat him to the punch). But what if he had gone through with it and Bobby had shared his cocaine with others, like those people seen leaving his house shortly before Harken arrived?
Harken proves himself to be one cold-blooded sociopath. His murder of Bobby is so vicious and calculated it couldn't be called a crime of passion. He even admits at one that he enjoyed what he did and was more successful at framing someone for murder than the protagonists. He not only has the ability but the willingness to hurt the lives of others for both personal gain and his sick whim. What kind of bad things has he done to rise to the top of the business world, and what did he do to get away with them?
