I think there are kinks that tend to reflect some aspect of your personality or upbringing, like NTR or femdom, and some that don't. For a while I really hoped zako was in the second category, because it's hard to explain the appeal in a way that does not sound psychotic. But it's not quite niche enough or absurd enough for me to fully believe that's the case.
This might not be a popular opinion, but I think a lot of conventional porn is steeped in some degree of misogyny. Kuroinu my beloathed is probably the most popular blatant example of this. Men conquer, women are conquered. Sex is an atrocity inflicted upon women for their (our) perverse enjoyment, an inherently non-consensual act tantamount to violence. Women only have power or agency insofar as it allows the audience to revel in seeing it ultimately taken away. The sexual desire to watch powerful, attractive women get subjugated and humiliated is seemingly ubiquitous, more vanilla than vanilla. They really do hate to see a girlboss winning.
But the zako fetish isn't necessarily an exception to this. So why is it I fucking hate Kuroinu, but find myself able to enjoy other works that are equally cruel if not more? Because, in many zako works, there is an underlying sense of tragedy. Sure, the zako are usually evil, but in my experience it feels like a superficial excuse for violence. You can rationalize slaughter all you like, but your primitive lizard brain will cry out in despair even as it begs for more. Human beings are complex and full of contradictions. It is neither a secret nor a coincidence that the "heroes" of zako fiction are largely either boring or war criminals. The REAL protagonists, the ones the target audience is reserving most of their attention for, are generally the zako themselves. It's similar to slasher movies: I'm there to see some heads roll, to be sure, but more likely than not I'll also find myself rooting for at least one character to survive the onslaught. No matter how you dress it up, there's something sacrilegious about watching the destruction of something beautiful, and this is a core part of the appeal. I get a similar mix of emotions from Kuroinu (hate is not the opposite of love, after all), but the difference is that I think this reaction is largely unintended.
The real world has been largely patriarchal for most of recorded human history. In fictional worlds, henchman organizations are largely glorified roadblocks. So why does it hit different when the organization in question is full of hot women? There's many reasons, but a big one is that zako are doomed by the narrative twice over. They're underdogs that are ACTUALLY underdogs, when we're so used to fictional underdogs being destined for greatness. They are, more often than not, enemies of society at large. Enemies of human nature. Perhaps even enemies of the world itself. And, frankly, I have more than a couple bones to pick with all three, so there's something freeing and transgressive in rooting for them despite it all. I don't expect everyone here to agree, but for me the occasional zako victory is sublime, especially when done by an artist that I wouldn't expect it from.
