There is a reason people like this are called “Grammar Nazis” and not “Grammar Heroes”. Language is ever evolving and the way in which people speak - to me - is more important than the rules of grammar. Knowing every last grammar rule and applying them to your writing doesn’t make you a good writer. In much the same way knowing everything about playing guitar - for example - doesn’t make you a great song writer. The anger over grammar and the need to “correct” others or point out flaws is, first and foremost, a matter of EGO.