First of all , Fishmen are weak and arrogant. Take Arlong and his crew for instance. Arlong got clapped by Vice Admiral Borsalino and took Refuge in a random East Blue village, boasting about Fishmens' strength and torturing weak humans. If bro was sent to battle a Rear Admiral blud would sweat bullets. Strongest ( probably only) Fishmen pirate crew of that time , and the second strongest person(Hachi) became a gag character later on. But they have their pride. They would boast about their strength to weak humans who can't defend themselves.Regular Fishmen are stated to be 10x stronger than random humans , but that doesn't make them anything special. There  are various other races who have as good or better feats than them ( Even minks are way better than them , and are quite amiable). The strongest Fishman is like YC3 category, and is  Ace rival. Now it comes to their stupidity. Stupid Fishmen are easily fooled by any influence, and are toxicly conservative. They always claim Humans hate them , but they didn't do sh*t to improve the circumstances to begin with , before Otohime. They stayed in an isolated island far away from humans , and looting them whenever they get the scope , and hoped humans would not hate them. Hody Jones' agenda would have long been demolished without even the help of the straw hats if Fishmen were less racist to begin with. 
Now the most important part,their morale. They hesitated to give blood to Luffy , who as well as liberated their country. What is the motive their? Fishmen simply hate humans, and cries when it's the other way around. The case of Fisher tiger was very distant from Luffy. He himself refused to take blood  because of hatred.  He deep down loathed humans. And then it comes to their habit of eating fishes. They are 'Fish'men , they look like FISHES. It doesn't make sense that they are not related to fishes , it's writing inconsistency or they are just lying. And do I need to include they are freaking ugly? 

This is why Fishmen exist to be hated.