You would be right, my friend. Antagonist-kun’s hatred for fish is connected to his tragical backstory. Anyone who doesn’t want spoilers for the eventual episode summary that will contain this, don’t read the section under the cut!
Antagonist-kun was the son of influential politicians and therefore lived in a rich environment for his entire life. Back in his childhood he was used to being spoiled and one day received a goldfish which grew on him a lot. He took it with himself everywhere along with its huge bowl, even. He was a problem kid who was allowed to do as he pleased in any environment due to his parents’ influence; he often got into conflict with the other kids and didn’t have any proper friends except the goldfish. One day his parents had to go on a trip overseas to participate in a conference regarding the protection of the environment; the two of them, being heavy capitalists, represented the side opposed to environmentalists. Due to Antagonist-kun’s parents’ well-known excellence in debates as well as their wealth, it was predicted that the two of them would turn the conference around (as they were planning to do), so it was crucial for them to go. They took a ship for the journey and left Antagonist-kun at home with a nanny. The next day he received news about the ship being blown up and leaving almost no survivors. His parents were confirmed to be dead. The explosion was discovered to be due to a bomb inside the ship, but whose doing it was remained shrouded in mystery. Upon hearing this, Antagonist-kun quickly developed a deep hatred for anything related to large bodies of water and even flushed his beloved goldfish down the toilet. He lived a few years with distant family members, then, reaching legal age, he moved in the countryside on his own and has been living off his parents’ remaining money ever since. He takes his anger towards his terrible fate out on fish, as they remind him of the tragedy as well as the better days of his childhood.