Yeah but it doesn't make sense to me. If a race has become devoid of negative attributes, how could they be so cruel as to leave Armus alone like that to suffer for all eternity? Especially if it's someone they just happened to come across. That's even worse, because that would imply Armus wasn't as evil before as when they left him. I understand that they would still have the negative attributes before they hypothetically met this Armus fellow, but once they rid themselves of their negativity and evil, it would seem quite cruel for them to just leave him there suffering and not try and fix the situation, or fix him somehow.
That's assuming it was intentional. If they just cast their negativity aside, it could be drawn to Armus telepathically just like the connection between Odo and Sisko, Dax and Garak in Things Past.
The history of armus only makes sense if the people who left him did so either before he came into consciousness, or before they were aware of his consciousness. That way, they are completely unaware of the suffering they have caused and their goodness is still intact. If they met some being, and turned him into an evil pile of sludge, and left him there to suffer, that goes against the whole idea of them having no evil within them. They would be total assholes, regardless of what kind of person armus was before they met him.
When Odo was first discovered, Dr Mora didn't know he was alive. Something similar could have happened.