Why are people so incredibly rude?

It's a known fact that in this world there are people you won't fancy. I know this very well, but why are people so rude? It doesn't matter how nice you are. You can have a really nasty attitude and still have manners. What I want to know is how come when I asked asked "Is anyone else pissed off about the how their teachers treat them" about 3 or 4 out of 5 people who answered, had something rude to say. One guy said that I sounded "spoiled and immature." He then went on criticizing my spelling, grammar, etc. This made me upset. He called me immature and he's one acting like a kid. saying that i sound like those negative things. If he had nothing positive to say I would've preferred that he said nothing at all. Also, so what if my grammar and spelling is off. I am very articulate and get very good grades. I know when I can and cant write incorrectly. So if i dont feel like spell checking everything I write and everything, SO WHAT! Piss off & hop off! I cant stand people who judgemental