Yeah, the thing is that judging her response without knowing what she's responding to makes us all look like fools.
There is always the possibility that this could be unwarranted, but hiding what might have provoked her causes us to rely on history ("she's never treated me like that, so they (or whoever took that screenshot) must've done something wrong") and bias (obviously we like her more than [who she was responding to]), which pretty quickly nullifies any argument one tries to make against her.
Personally, I've only seen Ema get mad on stream a handful of times over the years and it was always the same thing: someone didn't know when to stop bugging her, even if she told him to. The few screenshots I've seen like this are always a similar story: they asked a question and didn't like the answer so they kept prying about it OR they got super weird like that Edward guy did.
The fact that she writes "you bore me to death" leads me to believe this person wouldn't stop talking about something Ema either didn't want to discuss or had already given her final answer on and that she says, "I don't want anything with you just leave me the fuck alone," tells me that she wasn't being 'selfish,' but that whoever this was made her uncomfortable enough to do so. This girl will usually laugh off marriage, breeding and all kinds of sexual jokes like they're nothing, but this person actually repulsed her. Think about what must have been said for that to have happened.