[Morwen]

For me, Aredhel's mistake with respect to Gondolin begins with her decision to go with her brother when he reveals to her that he has plans to build a secret kingdom in a hidden valley and never intends to leave. Given that wandering far and wide is something that she loved since Valinor, it didn't require elven foresight to see that a cloistered existence was not the best thing for her.
I think that her decision, well demand really, to leave is undeniably selfish (she is putting her needs above the safety and security of an entire kingdom). But I do sympathise with her a bit at this point. The need to wander is a longstanding trait for her. It is not something that suddenly manifests itself as a result of being in Gondolin.Having made what is for her a bad decision, she is stuck with either living it (which given elven lifespans would be a daunting prospect) or selfishly demanding an out.
Having opted to leave Gondolin I do think that she ought to have stayed gone. I've a hard time sympathising with her here. She perhaps (mistakenly) thought that she could be happy in Gondolin the first time. Did she think the second time round would be the charm? Even if Eol had not followed her to Gondolin, would she have been happy there indefinitely (or at least until her evil seed child betrayed the kingdom)?
Ultimately I don't think she is to blame for the fall of Gondolin. I think more direct blame can be laid at the feet of Maeglin for his actions and also Turgon for his inaction. I do agree though that Aredhel is an interesting character.