Just remember that Role-Play does not mean "Role play a good guy" it means be "In-Character" and evil people should not be prosicuted in a GAME for being a bad person, because in real-life there are bad people as well.
I feel I should start my post from a completely diferent matter, but you'll have to pardon me on this one, seeing as this is just too hard to resist:
Yes! In real life there are bad people as well. And in similar manner
as well are they prosecuted for being "evil". More or less that's why you have jails. You're actually making arguments against yourself. What happens when someone sticks out of what would be perceived as "average" member of a society? He would have fingers pointed at him. If he speaks differently than others, this is what will happen. If you decide to use modern language, additionally with no respect for actual grammar ("What
are you up to", not "What you up to"), this is what is going to happen.
Now going to what I should start with: Your post makes absolutely no sense. You go on about how people pick on you for speaking in one way and not another, and then you switch to something about "evil" characters. Is that supposed to make any sense? Is evil character by definition one that speaks in weird dialect? I really don't see why culprit cannot speak in any way other people would. Rogue can be well mannered just as well as he can have tongue of a blacksmith or brain of a common peasant. Alignment has nothing to say here. You can role-play ill-bred character that doesn't do well with other people, but you don't indicate the above with "What you up to?"
I won't say you have no right to complain. Sure, pointing fingers at your role-play may be too harsh. However, only to some point. You should look after how your character speaks so that you don't annoy other people. Try to stay away from slangy'ish language (not only obvious like 'yo'). If you want to give some character to your speech, actually work on it. Good intentions alone won't cover all that is needed to be done in that direction. You can actually butch the grammar there if you want (just avoid sounding awkward), so long as you don't try to sound modern.
For example you can replace "What are you doing?" with "What do they?" and no one should complain. Though if you decide to do it, better make people familiar with your speech all the way. If you'll just switch between normal and self-generated, you may confuse others.
Now what I have to say is, my Planeshift career didn't actually "evolve" during CB. It did during early MB, where it was completely okay to use modern speech. But even so, it does sound pretty dull when you hang around people that
at least use normal English, and suddenly you see someone who obviously doesn't care. That's how I see it when playing Planeshift today. Also, that's how the community is. People are much more familiar to usage of normal and varieties of non-modern language so you'll seem awfully out of place if you do otherwise. At least try to avoid it.
P.S. "How be ye!" sounds oh so much more dwarfish than "hello" :P