Well being a social worker gave me a differing prospective in that some human beings would be worse that most animals. I particularly remember an infant I rescued after the landlord called us up about the abuse he was experiencing. He was a poor unfortunate child. His mother and father were mentally ill so he would likely never be adopted since most mental illness is inherited; especially the schizophrenia they both had.
The landlord was noticing a large amount of flies and awful smell near one apartment and realized that the couple was not using the trash cans a few buildings beyond their front door and merely tossing items out the door "willy nilly." So, he placed a large trash can next to their front door with a sanitizer to eliminate some of the smell, then continued to monitor the situation. Much to his dismay, the items seemed to never hit the trash can and still were tossed about loosely causing vermin.
Furious, he knocked on the door of the apartment he had rented to them. Eventually because of his persistence they opened the door. He was astounded to note the same situation permeated inside the home. And worse still he noted a fenced in area in the middle of the kitchen where there lay a mother dog, 3 pups, and a naked toddler eating from the dog dish.
He immediately notified us and I investigated. I removed the child for his safety. We attempted to do intervention but the parents were not capable of understanding. By the way, I haven't mentioned the roaches on the floor that would crawl up the sides of the food dishes and they would scoop them aside and continue to eat. Didn't mention three cats who were chasing mice into and out of the cabinets where they kept their cooking pots and pans. Well, I could go on about this environment.
Worse still, children like him could not be severed and placed for adoption because "insanity" is not deemed a legal reason to stop people from becoming parents. As an aside, these people had 4 daughters also that were removed shortly after their birth and placed into care. Why, because no one could stop them from giving birth either.
So if you asked me I'd answer, "No" all people are not suitable parents. Humans are animals when you come down to it and the most dangerous kind. Only humans kill for territory, sport and notoriety and not because they are hungry and need food,