Carl's parents seem to be abusive towards their son at first, but when he is in actual danger through outside forces, it turns into a Papa Wolf and Mama Bear team up.
Or, Carl's parents seem neglectful at first only because they are Hippies, and as the story progresses, it becomes obvious that they all love each other very much.
Carl claims he is brutally abused by his parents, but it's all a lie for attention.
Carl is an Enfant Terrible who abuses his parents, with the threat that he'll call social services and lie about being abused if they don't do what he wants. Finally, his parents snap and start disciplining him in ways that might seem abusive, but after the way we've seen Carl torment his parents, it comes across more as Kick the Son of a Bitch. At that point, the parents reason that going to jail is preferable to another minute of being tortured by the little psychopath.
Carl's parents are trying to discipline him, but his constant and escalating antics finally push them too far and they resort to more drastic measures.
It is found out that Carl's parents were giving him Tough Love, and it made him a stronger and well-behaved child.
Carl's parents were having a bad day and soon mellowed out at the next.
Discussed: "That villain must have been abused as a child."
Conversed: "What is with all this stuff? If they did that to their son in Real Life, they'd be locked up!"
The constant abuse from his parents leads Carl to believe that society is meaningless and he is either Driven to Suicide or grows upto be the Big Bad of the current story. His parents end up being portrayed more sympathetically: they had a more depressing backstory and were just too critical and both were a bit of a Knight Templar towards their son, making them much stronger and better persons.
Alternately, Carl's bruises just couldn't not be noticed by someone else, and abusive parents were quickly deprived of parental rights, before even causing anything he considers really severe.
The abuse drives the child to become a better person than he would have been, and makes him happier and more independent. It was his parents's plan all along.
However, neither Carl nor his parents are responsible for what they bore. So, none of them is more sympathetic in the end, and Carl is still a woobie.
It still made him think that it was normal for him to be hit.
It also makes them struggle with controlling their bad memories and their reaction to them resurfacing.
Alternatively... He instead grows up to try to be different from his abusive parents and becomes a loving and caring father, raising/becoming the next hero.
Or... Someone calls child protective services, and, while the parents are serving time (or whatever they do after CPS does their job), they see other parents treating their children right. After a while, they see the error of their ways, and do what they can to patch things up with their child, tearfully apologizing for what they've done and explaining why they did it.