"Maybe some characters just gain pleasure from causing harm to others." That is not the same as being evil. In medival times torturers worked for the king, and weren't seen as evil, since they punished the prisonners. I am referring to the definition of evil as something morally wrong. In order for something to be morally wrong it has to involve other people as moral standards depend on society. Therefore you can not be evil by yourself. Or you can, but then only to yourself.
This means there is no such thing as an evil character that has not be seen as evil by anyone. It means those characters are ridiculous, not evil.
If you define evil as being against the opinions of society, then you could still set out to do that- what If you wanted to be against the opinions of everyone else? What if you wanted everybody to hate you? Then, wouldnt you want to be evil? If, your character has a reason to desire a majority of people to dislike you, then, according to your definition, you your character has a reason to be evil. Now you might think that nobody would want people to hate them, but there are reasons and advantages to be gained from it. It's entirely plausible that somebody could have an ulterior motive for having a majority of people dislike them.
Having said that, I dont think thats what most guilds/characters mean or are looking for when they say they are 'evil'. But, saying your character is 'evil' could then be an awknowledgement that 'I think alot of people wont be happy with what I do; Thats ok with me'.
You seem to have the perception that everything is subjective and merely opinions of others, but I think that 'evil' characters are going to behave instinctively in a different manner to 'good' players. If the society has a clear set of morals and expectations that it follows, you could confidently predict that your character will be labelled 'evil' from their actions; therefore why not state that beforehand? Its a trueism to say "Characters will do what they perceive to be best" but I think the evil characters care less about other's opinions, and inherently have less respect for other characters then 'good' characters do. So I think there is still a measure of objectiveness about whether a character is good or evil.
So if you create a character/guild that has had no interaction with any society whatsoever and you yourself claim it to be evil, then you are wrong. You're whatever people think of you. If your guild for instance manages to wipe out obstacles for mine, I won't consider you evil.
If you're werent interacting with society, then you're not doing anything against other characters, and so It would be impossible to be evil in the first place in any sense. Its assumed that any character or guild in the Planeshift world is going to interact with it.