I can't believe I am saying this but the more I know about Arlong, the more I feel sorry for him.
When he was first introduced way back in the East Blue Saga, he was shown to be a total creep and he was completely despicable. After seeing the way he betrayed Nami, I enjoyed seeing Luffy beating that guy to a pulp. And then right after Luffy and his crew crushed the Arlong Pirates, I thought that will be the end of it and I had no idea that we will learn more about Arlong hundreds of chapters later.
Then much later at the start of the Sabaody Archipelago Arc, we see Hatchan again. After the Straw Hats enter the Archipelago, we learn about the discrimination towards fishmen and merfolk, which will be key point much later on.
Then after two years, the Straw Hats finally arrive at Fishman Island. Before they arrive at Fishman Island, we were thinking that the Straw Hats will save the island from pirate trouble by making it their territory. However, we soon learn that island is already under the protection of Big Mam and the Straw Hats will learn something of great significance, the history of Otohime and Fisher Tiger. Right before the New Fishman Pirates were introduced, I had no idea that Arlong had a fan club and some people were thinking that their boss, Hordy Jones, is a relative of Arlong. Later we see that Hordy Jones is only an admirer of Arlong since he also shares his views of humans.
When Jinbe finally starts his explanation about letting Arlong loose into the East Blue, we get to see the history of the Sun Pirates. Arlong always was a jerk since his younger years and even Jinbe hated him. In a battle with the Marines at the end of Chapter 621, Arlong told Jinbe to leave the unconscious marines alone, which surprised me. But then in the next chapter, we see how arrogant Arlong has become going as far as to suggest killing more humans to instill fear into them. Then we were introduced to Koala and Arlong was immediately hostile towards her just because she's human even though she was a former slave who suffered the same cruel treatment like the fishmen who were enslaved. Even though most of the Sun Pirates warmed up to Koala, Arlong remained being a jerk to her, saying that she is just the same as all the humans he known and hated. And after Koala returned home, the Sun Pirates were soon ambushed by the Marines (who were tipped off) and the ensuing battle cost Fisher Tiger his life. The fact that Tiger was betrayed further Arlong's hatred to the point that his hate hit rock bottom.
As Hatchan said, Arlong always held bitterness towards humans. And it was because Arlong only saw and knew their dark side. I understand his reasons for hating humans, but he was taking his hatred on the wrong people.
PS: There is something I have been wondering. Could it be that Mohmoo's sudden reappearance be some kind of foreshadowing? Mohmoo was part of Arlong's crew after all and after he reappeared, we were introduced to some fishmen who look up to Arlong.