I've found more often than not that if someone can abuse twitter they will, but in this case I tend to side with Rob McClure. There's this expectation that artists have to take negative feedback without retaliation because to do otherwise would be childish and entitled, which was all fine and good before social media was a thing, but now people feel entitled to post some really hurtful stuff about celebrities on twitter, emboldened with the belief the celebs themselves won't find out about it, and it leads to some pretty toxic places.
The original tweet wasn't the worst thing that could be possibly said, but the tweet specifically pointed out his face and was phrased in such a way that it was clearly negatively commenting on his appearance (and the original tweeter's later responses to him clarify that it was a dig at his appearance), and he has a right to be hurt and to point out that he can see everything people post about him on social media. While his comments about her grammar were a bit petty, I don't think anything he said constituted "going in on this girl".
I honestly wish more celebs pointed out that they can see all the stuff people post about them, especially if they're tagged. I remember right after the Tony's I was searching Santino Fontana's name on twitter (don't judge me, I'm a big fan, okay) and I found several tweets that he had been tagged in, saying that they hope his wife has a miscarriage. I remember just being appalled that someone would not only say such a thing but tag him in the tweet to make him read it.
I think a lot of people on twitter would benefit from a reminder that everyone can see the hurtful words you post in a moment of anger, and hopefully Rob McClure's response will provide that reminder, if only temporarily.