He deserved that. This still has yandere vibes, though....
Yeah, looked at a slightly different way (and I’m not saying this is how it should be, or is intended to be, read), she made several assumptions and instigated violent action on very slim evidence.
That is, the writer made her be right that a) the hate mail was a real threat, b) the lurking guy was the same stalker who sent the hate mail, and c) messing with his camera and beating him up, and then essentially lying about being hit (it’s pretty clear that the guy missed with his punch) was all OK.
In other words, he did turn out deserve that, but it’s not clear that she had any reason to know that he deserved it. Of course, by saying, “I’ll ruin you too,” Attacker Guy gives his own game away, so for now it’s all good. But questions remain . . .