I'm feeling very confident that every Evanuris was a monster. The Dalish portraits of them are all completely false and the specific ideals and images they associate with the Creators might not all even be based on anything the Evanuris did, or are so loosely connected to the real versions of them as to be basically useless in trying to determine what they were really like. Just to speculate, Sylaise could end up being like the goddess of dungeons or something. Or maybe the goddess of fire, known for burning people. Or perhaps she's known for creating terrible structures in the same way Ghilan'nain is known for creating horrifying creatures. The Dalish religion is apparently largely based on a combination of nostalgia, and the various aspects of their present day civilization. That is why Fen'harel is remembered the way he is, their memory of the fall of Arlathan is colored by their memory of the fall of the Dales. June is known for being a craft-worker and builder, but that is all stuff that seems to be relevant to the current Dalish Clans. He wasn't known as that before, he could have been a mad scientist like Ghilan'nain, just in a different field of expertise, like creating weapons of mass destruction or something.

The Forgotten Ones were likely also evil. We only get a glimpse of them but it suggests they were only interested in power. I think the idea was that everyone was evil back then, because that's just what the moral standards of the time were.