I am not a particularly nice person.
Not exactly a surprise to most people who've seen me discuss things, I'm sure. If someone gets on my bad side in an argument, "aggressive" is about the most gentle tone I'm likely to deploy.
I've told people to fuck off before. I've questioned their intelligence, I've mocked them for their mistakes, and I'll cut off conversation if someone comes across to me as dishonest, if they seem unworthy of a civilized discussion. Usually I do these things with people who have their own asshole traits on display, some of which may even exceed my own - Cultists of Gnu are able to manage this reliably, to the shock of no one, but it would be dishonest to say I snarl only at the Cogs. I've told off classical theists, conservatives, "progressives" and more.
What I don't do, however, is bullshit.
If I tell someone off and call them dishonest, and therefore I don't find them worth an extended interaction with - I'm not digging around looking for the magic words to say that will rile them up the most, or make them look bad. I'm simply dropping formalities and being direct. Likewise if I call someone stupid - that's not a mere barb, that's me deciding that there's no point holding out hope anymore that I'm dealing with an intelligent individual, and this plain evaluation of their capabilities usually is directed at people who mistakenly believe themselves to be very smart indeed. On the flipside, I don't tell people who I think are dishonest that I also think they're stupid unless I really believe that about them - and more often than not, the worst I can say about dishonest people is they aren't quite as good at bullshitting as they believe.
Important to realize here is that even when I flip someone off, I generally am not acting hastily and out of anger. It usually means I've evaluated the situation, I've reached a limit for this, that or the other thing, and now the tone of correspondence is about to change because the previous way simply was not sustainable or worthwhile. This goes in both directions, by the by - I've snarled at people who turned out to be quite decent, and I adjusted myself accordingly. Others shaped up. Others were civil and even friendly to me, and then did something which set me off, and I changed tone decisively as a result.
Being "not nice" doesn't mean "being unfair" or "not being friendly at times, even often".
Part of this is because I'm not interested in the modern kabuki theatre where I think someone is deceitful, but I'm worried that people will think lesser of me if I come out and SAY that, so I hold my tongue and try to trip them up in circuitous ways all while feigning civility and friendliness like some kind of wannabe aristocrat. I don't care what people think of me in the main.
I think this kind of 'let's be nice' attitude too often leads to rewarding the most obviously full-of-shit among us - people who spin words, twist knives, and generally behave irritatingly, but in that technically 'polite' way that serves as a shield for their being called out. Conversations with these people devolve into concern trolling, and generally 'being a prick but in an indirect way in the hopes of provoking'. I prefer to skip the formalities and tell someone what I think of them bluntly when it's warranted.
Worse, I think 'being nice' is all too often related to 'being politically correct', which I consider a kind of cultural poison. It's not 'nice' to talk about various cultural realities, even in a conversation broadly covering those exact topics. If we're discussing LGB rights and culture, it's not 'nice' to talk about sexual habits bluntly, moral failings, and for God's sake certainly not disease outbreaks. If we're "having a conversation about race", then everything from crime rates to negative views of culture to (again, for God's sake) standardized test results is also not "nice". This is a shifting battlefield many times, with various sides trying to out-nice and out-accuse-others of not being nice, presumably because some people out there will punish the person who is collectively judged to be The Least Nice.
Not a game I wish to play, ladies and gentlemen. Nor is it one I can recommend. I don't endorse just plain being an asshole unprovoked, but I likewise do not endorse maintaining a civil conversation with people you sincerely think don't merit one, or letting someone you're quite sure is full of shit continuing to talk as if they were, in fact, utterly sincere and not yanking your chain, pushing your buttons, or the like. I suspect that if everyone were a little less "nice", we'd actually make headway on a host of problems.
At least after the fistfights stopped.