User blog comment:Mazer947/Why does Duncan hate Harold so much?/@comment-5231983-20200204015720

TDI's premise is a bunch of teenager archetypes interacting with one another. A very common teenage character cliché is the bully character, and another is the nerd character – These two often are at odds with each other, and Duncan and Harold's relationship is meant to portray that. Remember that Duncan has spent a good part of his teenage life in Juvenile Hall, which isn't really an easy place to be. When there's no wardens watching the kids, there's a power vacuum that someone has to fill. If you're the strongest, you fill that power slot and gain everyone's fear. By gaining their fear, you gain their disrespect.

Duncan is used to operating in that sort of power dynamic where he has to assert social dominance over others in the easiest way he can. You can see this in the first episode with his interactions aside from Harold – He's commonly seen flirting with Heather, which establishes him as a "player" type of person. He bullies Noah as well, by grabbing his lip in front of everyone and threatening to dangerously pierce his lip with an unsterilized needle he just happened to have on him. This quickly helped Duncan establish his bad boy persona; We even see Tyler and Gwen briefly discussing Duncan's reputation as someone you wouldn't really want to be around.

Duncan's treatment of Harold is the result of Harold being an easy target for him to assert his dominance on. Harold is weak, eager to make friends, and easy to make fun of. Duncan capitalized on this opportunity by immediately turning on Harold, establishing that the two of them will never be friends. And all of that is just in the first episode. The trend continues as Harold keeps being sort of a screwup, such as in the cliff diving challenge where he hurts himself pretty badly. Duncan develops his bad boy persona, which we know he did intentionally. He also gets to have some fun doing it because he enjoys picking on people who are due to the conditions he's grown up in.

Essentially, this continues because Harold never stood up to Duncan until later in the season, and even then he does so indirectly, so Duncan isn't afraid to pick on Harold even more than he was already doing. tl;dr It's a toxic "alpha male" power dynamic concept.