Hm, I don't blame Gu Ze, it makes sense based on his character so far.
1.) The girls tried to use him to deliver the letter.
2.) She was overconfident that she'll be liked back without actually knowing ML at all.
3.) She mentions that she only likes ML because of his good grades and cause' of long-term rumors. And 'o I'm so great and pretty and he must like me back'. That's shallow af.
Gu Ze didn't want to tolerate her BS and just told her the truth. And when she didn't listen to that he got mad because he (is crushing on ML) and didn't like the idea of such a shallow girl ending up with him. So yes he went overboard, but i think it's a human reaction. Remember that he's also the guy that rigged the ML's bike a long time ago and caused him to crash. Gu Ze isn't a saint and never will be. People he don't like he doesn't act cordial with, like his dad, for example.
I like Gu Ze for that. He's not the kind of 'saint MC' that just lets someone give a letter to the person they're crushing on then looks onto the scene with sad eyes like 'o i liked him after all now I regret but it's too late'.
That girl still has the complete ability to go and confess if she wants to. She should have just done it in person to begin with. I'm not saying it was the RIGHT thing for Gu Ze to rip the letter and threaten her, but based on his character it makes sense. Plus it's kind of obvious that this is gonna cause trouble for Gu Ze soon enough.