Except it's not violence. Don't try to enforce your own personal code of ethics as the one most of society goes by.
Replace with “cheating” and the point remains the same. Him betraying her explains her pain, but it doesn’t make it right for her to do the same. If the relationship was already over for her, she could have ended it before kissing someone else. Hitting back doesn’t make you better; it just repeats the same behavior. And for a serialized manga, I expected a little more intelligent drama, not the same cheap NTR doujin cliché.
That is why we said women should be kept away from writing seinen—they might produce abominations like this.