If cats are so fucking equivalent to humans, how come thousands get burned in incinerators every day, just in this country alone.
You seem to be irrationally fixated on the wrong problem, and it’s a singularly silly hill to die on.
The problem is not “OP is anthropomorphizing her cats”. The problem is “her cats used to get along and now they don’t.”
Hey, easy with the pronouns! I’m a he! 
Thanks for all the input guys,
Cats are about due for a check up anyways, so I guess I’ll mention their behavior to the vet when I take them in.
Because humans (and cats!) like killing things. We kill for fun, and for convenvince, or just out of boredom. I mean, thousands of people die every day, too. It’s not like we lose any sleep over that. And it’s not like the cat would be nicer to us, if they were all tigers who out numbered us.
How any of that is a refutation of the biological reality of our neurological and behavioral similarities is unclear.
How many humans are gathered each day and killed against their will in a fiery furnace? Not many.
If cats were like humans, they would figure out how to survive. They would have evolved and developed the usage of tools and built structures. They would have developed a written and spoken language. But they haven’t and they aren’t. They’re cats.
We had a similar situation when one cat ended up at the vet’s office overnight. I guess he picked up a new scent or something, because his brother, who had previously spent every waking moment with for the previous ten years, spent the next two days trying to kill him and generally acting like he had no idea who this new cat was.
I’m guessing he picked up some other cat’s scent somehow. It’ll pass.