I don't believe the issue people have with the idea of Izzy being Ed's abuser is because fans are unwilling to view Ed as a victim of abuse or Izzy as capable of being an abuser. I feel like it's a more simple answer of "people don't agree with that interpretation because there isn't enough to substantiate it."
With Izzy and Ed, it's important to understand the difference between conflict and abuse. (I'd highly recommend "Conflict is Not Abuse" by Sarah Schulman!) A lot of the time in highly volatile relationships, we're quick to assign abuse to them and to figure out which person is the perpetrator and who's the victim, but often times they're just conflicted. This is why you'll often hear Izzy stans describing their relationship as mutually toxic, not mutually abusive (which isn't real)
The simplest definition is determining whether the relationship is based in Power Struggle or Power Over. Abuse isn't based off of individual actions, but an exertion of power. Both Izzy and Ed commit acts as part of a power struggle towards each other, with Izzy's antagonism of Stede and utilization of the navy, and Ed's manipulation and physical violence of punching, choking, and mutilating. (Yes, physical violence is an expression of power!) There's a back and forth here with both having moments of forcing the other to stay, and neither of them being the picture of a healthy relationship. With them, there's also the added element of Izzy's privilege as a white man versus Ed's position as Izzy's boss which are both significant power imbalances that factor into each other's toxicity.
The important part is that Ed's feeling negatively towards Izzy doesn't equate to being an abuser. Izzy vaguely threatens Ed ("Edward better watch his fucking step") but this is also within a context where Ed just choked him. Izzy had called the navy before, yeah, but that option isn't available for him anymore, and Ed still has an advantage of being the only thing keeping the crew from throwing Izzy overboard with an anchor anklet. Arguably, Ed holds more power over Izzy in this specific instance. Rationally, there isn't an immediate threat here, but Ed still responds as if there is.
Ignoring all that, the main part of this is that Ed's Kraken response is indicative of the other person being an abuser. "If someone reminds Ed of his past abuse that much then it must mean that they're in the wrong!" But that's not how that works. Take this passage from Conflict is Not Abuse as an example:
This is also not how Trauma™️ responses functions. Ed, incontestably I hope, has some form of PTSD/c-PTSD. The very defining aspect of PTSD is that a person experiences a traumatic event that they continue to not recover from impacting their day to day life. Often people going through traumatic events will struggle for a bit before getting better, but not everyone does that. When the symptoms continue or even grow worse, that's when we identify PTSD.
PTSD reactions aren't rational. Especially when it comes to c-PTSD, the ability to gauge and respond to threats is damaged. You become easily triggered by things, often seemingly unrelated to an outsider, that reminds you of those traumatic experiences and throws you into survival mode. People with PTSD and who have suffered from abuse are not able to rely on gut instinct alone. That meter has been damaged where the threat alarm is going off at a hair trigger, leaving the survivor of trauma the options of avoiding those triggers completely (nearly impossible) or learning to suppress that. This can also leave survivors of abuse especially prone to revictimization. When every action someone takes looks like a red flag, you learn to tune out that alarm bell, including the times when it's not an overreaction.
If we assume that Ed reacting with the Kraken is indicative of the other person being an abuser, then that'd mean we'd have to assume that Stede's crew was a threat. Ed killed his dad and Ed killed Lucius, so naturally, Lucius must have been abusing Ed. You can extend it as far as Stede as well, since David Jenkins described Stede's rejection as "deranging" Ed, and Ed while acting as the Kraken is tossing out Stede's shit and marooning his playthings. But we know that Lucius only had the best of intentions for Ed, and we know that the crew is too incompetent to hurt Ed.
So what the fuck is going on with Ed?
Simple answer is that Ed feels threatened. Ed's scared. He doesn't feel safe. When chronically traumatized people feel unsafe, they react in defense, including in ways that are maladaptive to themselves, and harmful to others. One way to conceptualize it is through the Internal Family Systems (I wrote an analysis through this lens once!) Within IFS, you have two basic categories of Protectors and Exiles. Exiles are the part of us who hold the pain and shame of our trauma, usually from childhood. Protectors are the parts of us who develop strategies, usually maladaptive, to protect us from that pain. I'm severely simplifying, but I've found this site to be helpful with breaking down the core concepts.
We can think of the Kraken as taking on the role of a Firefighter. The "break glass in case of emergency" protector who comes out when we're in "danger."
Firefighters will do whatever they need to when it comes to stopping the danger, even pushing us into far more fraught situations. This can include things such as binge drinking, self-harm, serial cheating, and other actions we wouldn't rationally view as safe, but things like drinking can numb the pain, self-harm creates feelings of control, and cheating brings reassurance that you are wanted. They're quick fixes with a disregard for consequences in the moment, but they're actions done to "protect" you from danger.
But like I said, trauma can really skew your sense of danger.
Just because someone triggers your PTSD and brings out your greatest threat response, doesn't mean the threat is validated. In the same way flinching when your partner casually reaches out to touch you doesn't mean they're at risk of beating you.
Ed's response to Izzy could be an overreaction to Izzy's vague verbal threat, or it could be a solution to quelling Ed's fear of abandonment, or something else entirely. It could be reminding Ed of his father, but it doesn't mean that Izzy is an abuser. Especially within a context where we've never seen Izzy pose a physical threat to Ed, where the closest we got is him summoning the navy on his white boyfriend, and ensuring that Ed was not harmed in the interaction. Ed's use of physical violence against Izzy isn't proof of Izzy's abuse, no more than it would be for Ed throwing Lucius overboard.
Something Sarah Schulman goes into detail about with the necessity of drawing a difference between conflict and abuse is misidentification of abuse stemming from supremacy vs from trauma. With supremacy, you can't just trust your gut feelings because that ends up with things like white women having moc murdered. Traumatized responses are ones where past victimization interferes with our ability to differentiate between abuse and conflict. These can often overlap with clear borders, but there are differences, of course.
The reason people don't view this dynamic as abusive isn't from an unwillingness to see Ed as a victim, but from knowing that he has been victimized in the past. The level of trauma he sustained as a child severely fucks with someone's head. Not metaphorically either, it literally causes brain damage and has been linked to an increase in likelihood of developing autoimmune diseases. Like, trauma can be so bad that your body just starts eating itself it's fucking wild the amount of damage it can do to a person.
Recognizing that Ed’s actions can be wrong, but still extending empathy towards his place as a survivor of abuse, is an act of compassion towards him.