The ZKK theory didn't directly make Zoro fans more toxic.
The community response towards the theory and other things concerning Zoro made his fans more toxic.
When you reject things for no good reason, then you should prepare for toxicity.
For example, "Zoro will never get CoC" or "Zoro won't even get to see Kaido".
These sorts of things create toxicity, and as a result, Zoro fans will reply in a toxic manner too.
And when they reply like that, the community complains about their toxicity.
It's a funny cycle, but yeah, it will never be solved.