No blame falls to you for lowering the argument to insults; It had already been lowered by someone else. The idea that a person would need to keep to the high road when the other person has resorted to insults, otherwise they're equally to blame, is ultimately an idea that people who aren't part of the argument can easily afford to entertain. If you only cash in hits without taking any swings, you'll just end up being a target.
That said, of course in an online argument there's (usually) no risk to just stepping back and not responding to provocations. That doesn't excuse the both-sides-must-be-guilty attitude though.
I did not say that both sides must be guilty, my original post simply asked both parties to stop arguing and I said that and I entertain this argument because it is how I behave in situations like these even when I am the one originally being insulted. I understand what you are getting at, and in many many cases doing the whole "there is good and bad on both sides" schpiel VASTLY undercuts the wrongdoings on one side simply because the other side lashed back, but as you mentioned yourself it most likely would have been fine here to simply step back and not respond.
Ultimately, that original comment was lighthearted and I simply wanted the people who were bickering like children in a comment section where we had previously had a good discussion to cut it out. Yes, one side started it. Yes, "both sides"-ing a topic is often centrist and ignorant. No, I do not think that it matters that much here.
Again, I get your point, and I do agree with it on principle, but I am not going to do a full dissertation on the non-effectiveness of lashing back at your haters on MangaDex, and I will instead try to just go "please be quiet already".