Well, if you judge Ansem, the Seeker of Darkness and the Nobodies by their actions, then they are evil. Ansem has nearly plunged the entire world into eternal Darkness. The Nobodies hunt down Heartless to obtain hearts so they could be human again, and is willing to use anyone just to accomplish their goal.
However, if you judge them by their thinking, then it's a totally different thing: Ansem truly believes that everything should be plunged to the Darkness because he thinks that that is the best fo everyone. His knowledge had taught him that the Darkness is the best, and anything else doesn't matter (Which is why he said to Sora, "You know nothing. You understand nothing." So to Ansem, he was actually trying to do good for the world. After all, who is to judge good and evil? Who knows, that he might be right?
And to the Nobodies, why do they even care whether their actions are evil? They have no heart. They could no longer feel whether the actions are right or wrong. Their lack of heart leads to lack of emotions and thus altering their judgement on their actions. They do not think it was wrong even though humans think they were. And if we are to think even more deeper, they are a hundred times more pitiful than humans. Their lack of heart is much more unbearable than losing an arm or a leg. And bearing with that thought, naturally they would think that their judgements are right. All they wanted to be is to feel again, to be human again. Why should the humans stop us? This is why the Organization XIII seemed to be burning with hatred and anger, even though they do not have hearts.
However, if you judge them by their thinking, then it's a totally different thing: Ansem truly believes that everything should be plunged to the Darkness because he thinks that that is the best fo everyone. His knowledge had taught him that the Darkness is the best, and anything else doesn't matter (Which is why he said to Sora, "You know nothing. You understand nothing." So to Ansem, he was actually trying to do good for the world. After all, who is to judge good and evil? Who knows, that he might be right?
And to the Nobodies, why do they even care whether their actions are evil? They have no heart. They could no longer feel whether the actions are right or wrong. Their lack of heart leads to lack of emotions and thus altering their judgement on their actions. They do not think it was wrong even though humans think they were. And if we are to think even more deeper, they are a hundred times more pitiful than humans. Their lack of heart is much more unbearable than losing an arm or a leg. And bearing with that thought, naturally they would think that their judgements are right. All they wanted to be is to feel again, to be human again. Why should the humans stop us? This is why the Organization XIII seemed to be burning with hatred and anger, even though they do not have hearts.
