That's familiar. I have experienced another's vulnerability as viscerally disgusting before, and it can trigger attacking impulses. Thinking about it, at a certain level of vulnerability, the person's behavior becomes "pitiful," so to speak, and it seems like the person is trying to evince high levels of empathy from me. This seems terribly manipulative to me. It's even more the case when it's as a result of poorly thought out decisions on the part of the other person. Case in point - when I worked at this pizza place, there was this one middle-aged woman who worked the register. Single mom, trying to scrape by, she had a hard existence, no doubt. However, near the end of her shift, she would always get antsy about leaving early, so she could "get to school," especially when there was a long line of customers. The neediness began to grate, since I had my own things I was dealing with, and you didn't see me constantly pestering about it (instead, I'd be chronically dysthymic and drink too much). At the point, though, that I found out that "school" was one of those for-profit universities (largely a scam designed to prey on the hopes of the lower classes), that's when the impulse to attack set in. Even though I know it was a warped perspective, it seemed like she was trying to be as pathetic as possible, just to go make an even more pathetic choice. The feeling is where's your goddamn pride?
I also don't necessarily know if the refusing to stop after you ask would arise from that vulnerability, though. Sometimes, when I'm on a roll and heated, there's just a lot of righteous indignation built up there. So, if you were to ask me to stop, that angers me further, because I'm thinking "why should I be the one to stop when you're the one who started this in the first place?" There's nothing rational about this; it's simply that there's anger that needs to be burned off somehow.
Your second friend there brings up a different scenario in my mind - when I am trying to suppress a strong emotion because it seems unhelpful to me at the time, but the other person does not do so as well, so the empathic response makes it even harder to suppress one's own emotional state. The difficulty of self-control is frightening in this circumstance, and to reestablish self-control, it seems like the obstacle to that control must be dealt with. Again, the issues are familiar: power and control.