Whenever you write about anyone accused of a crime, remember that they are "innocent until proved guilty." If someone has been charged with murder, for example, say "alleged" murderer.
If you don't use words like "alleged" or "charged with," anything you write that defames the accused before his/her trial can be considered as slander or libel, and you can find yourself in troubles of your own.
If the accused receives a guilty verdict, you can then drop the "alleged," but you still can't say anything defamatory about the defendent unless you can prove it's true.
The next time you read a newspaper or listen to TV news, notice how careful reporters are to avoid condemning the accused, especially before the trial. They don't like getting sued.
This forum, like newspapers and television, is in the public domain, and subject to the same restrictions as any other public means of communication. Generally, it's not worth most people's time and money to sue someone who said something nasty about them on a website. But, as this thread shows, sometimes you can get bitten back if you bite first.
Whatever you say privately to someone, however, you can let fly whatever you like.