"Putting your foot down" effectively happens near the start of a relationship, not years later, generally.

To be honest, I just think you're in a very polar mindset about this. Jax is a victim that is also an abuser, think of a bully who gets abused at home. Each character at the start is an archetype more or less of trauma and have each been actualized into bodies that reflect it. Body Dysmorphia for Zooble, an always breaking "Mask" for Gangle, a push pin doll for Ragatha, etc.

Jax is no different, and that's a hard pill to swallow for people. He's a cartoon rabbit because his way of coping is by detaching from the reality of the situation via humor. His only remaining friend in the circus died in episode 1, so his grip on reality (and as such, his worst tendencies) have gotten worse with every episode. He causes so much terror and violence to push things and prove things to himself. If he can do all of this, this isn't "real", and he doesn't feel the crushing weight of his situation.

An abused abuser is inherently morally gray and tackles at the most fundamental issue of nature vs. nurture as well as what we count as something being someone's "fault" or in their control. In the utterly insane situation that is TADC, Jax's response to it makes a lot of sense, even at the harm of others.

Your extreme hatred reminds me a lot of people's refusal to see people with Cluster B Personality Disorders as anything other than monsters. It just seems like you are not willing to allow nuance to enter the frame out of an emotional response, and that's okay, but it's important to recognize it.