If you try to hurt someone and succeed, it's their fault?
This is pretty much the most basic case of fault.
I guess I view "fault" a little differently, in that I don't usually see it as a resource that needs to be shared collectively between both parties.
My take is this: Everyone is responsible ("at fault" in a sense) for whatever they had control over. Intent matters.
If someone intends to hurt me with words, and I am hurt, they are 100% responsible for their desire to hurt someone else. They're a bad person. If I get mad and yell at them because I'm offended, I'm 100% responsible for that behavior as well. It's not split.
If someone intends to hurt me with words, and I'm not hurt, that doesn't actually diminish their responsibility. They still tried. They're still a jerk. 100% responsible for the exact same actions as in the first scenario.
If someone isn't intending to hurt me, but is perhaps careless because let's say I had told them I get mad when people insult me and they forgot, and I am hurt... They're 100% responsible for carelessness. But they aren't the same jerk as in the earlier scenarios. And I'm still responsible for my reaction, too.
If they had no way of knowing I'd be upset... Well, then I bear yet another different sort of responsibility. And if they do/don't get upset, they likewise are responsible for their actions.
In reality this isn't always clear-cut and doesn't automatically tell us who is the jerk in the situation. Cultural context matters a lot. We're coming up on the anniversary of my mother's death; if someone were to knowingly mock my dead mother, most cultural context is on my side, and that guy will be seen as a jerk.
Other situations might be more or less controversial. I'm speaking broadly here, focusing on principles rather than specifics, because I think people are misconstruing my position as endorsing specific actions that I may not actually endorse.