You're having trouble looking at it from a perspective that is not that of the humans in the story. You're innately considering anything that eats humans as "evil". But if your protagonists are monsters, then human morality doesn't factor into the narrative evil.
To put it differently, just because Superman eats a steak doesn't mean he's the villain. In your story, the humans fulfill the same role as the cow in my example.
You are effectively able to reuse "normal" human characters from most-if-not-all stories for your monsters, because to the monsters, humans are a food source that is irrelevant to their own culture other than its cuisine, much like how humans eating meat isn't the driving narrative of each and every story about humans. Roméo & Juliet (to pick a random story) is not a story driven by a conflict between those who eat beef and those who don't.
It's really hard to make the antagonists distinct when they all have the same generic "serial killer" personality.
Many people who have killed are not necessarily killers, let alone psychopaths or brutes. Those are really not the same thing.
You are equating someone who eats a steak to someone who hunts and murders for recreational purposes. That is a massive conflation of different characters and personalities, all because you judge the characters by the singular "eats human" moniker.
You're going to need to stop trying to paint these characters based on how their prey would think of them, because the prey is not the protagonist (nor narrator) of this story. A narrative tends to focus its moral center on the protagonist, which is not necessarily a pro-human point of view. And even if it is, that does not mean that any view the protagonist does not share is therefore irrefutably evil. Not everything lives on the good/bad scale.
Given this, how can I diversify the personalities of my antagonist characters to make them more interesting while still keeping them villainous and antagonistic?
The short answer here is "by not painting them as black vs white".
The most interesting villains are those whose backstory is, to some degree, relatable. In other words, they don't do evil for evil's sake.
- Victor Freeze wants to undo the mistake that ended up putting his wife in what is essentially a comatose state.
- Magneto wants to fight xenophobic hate to every degree, no holds barred, due to his experiences as an incarcerated Jew in the Holocaust, effectively matching the furore that the Nazis opposed him.
- Thanos wants to end the dog-eat-dog nature of an overcrowded universe that's fighting over resources
- Walter White wants to finally build the kind of empire that he could've built years earlier, had he not backed out of the company he founded. He rejects how meek he had let himself become and overcorrects the other way.
- The T-1000 is just a machine doing the job it was built to do. It's no different from a watch telling the time.
- Gollum/Sméagol is coping with the equivalent of a lethal drug addiction, and cannot stop himself against all better will.
- [Memento] Teddy tries to both give the protagonist the pleasure of achieving their goal, and is trying to enact vigilante justice.
Or, if you're looking for example of monsters not being villains, watch Monsters Inc. Scaring small children is not a nice thing to do, and it technically features into the everyday lives of the monsters we see (for power generation purposes), but in reality it does not drive the monsters' character on a good/evil scale. Their lives and narrative roles are not defined by what they do for power generation.
From their own perspective, none of these villains would describe their own actions as evil. They are the protagonist in their own story. The only reason they are the villain is because we are watching a different story, one in which they oppose or conflict with the protagonist of our story.
Or, maybe a more apt analogy, you are painting your monsters as the Xenomorph from Alien. But compare the Xenomorph to Predator. Even though Predator, at face value, never communicates and does nothing but hunt, there is a huge difference in their character, as Predator is shown to be honorable and sapient (i.e. cultured, smart, rational), whereas the Xenomorph is only sentient (i.e. an animal with an instinct).