I'm on the same page as you I think when it comes to Flemythal. Flemeth has been one of my favorite DA characters since forever. With that said, I have never thought that she has noble/pure intentions. I just love how grey she is, ya know? She's a fantastic character. I hope there's going to be more of her or her machinations in future installments.
I love that Bioware has made so many characters in this series that fall into the morally grey alignment. They're complex. There's things to love about them, sometimes they do good things, but there's enough there to still kind of fear what their real intentions are. Morrigan is also one of my favorite characters. For largely the same reasons.
Yay, someone gets it!
I'm not hating on Mythal. And I don't 100% know that I'm correct. It's just that her desire for vengeance above and beyond what Solas has already done in the name of vengeance, makes me highly suspicious of her. I can see she cares about the modern world to some degree. What I don't know is whether that's because she cares about the people in it or because she had to keep a certain balance over time until Solas woke up so she could achieve her ultimate goal of vengeance.
It's not just Mythal's vengeance, it is also Flemeth's. Mostly Mythal's, though, I'd argue.
So here's a question for you then: What happens when the mother/protector figure is betrayed and the people she was mother and protector to no longer exist, or are hurting somewhere?
Also, if you were nearly impossible to kill and people you had long worked with and possibly trusted to some degree struck against you, wouldn't you be angry? And what if you had millennia to stew over your vengeance, just like Mythal has? It doesn't mean every one of your actions would push for that goal, or that you'd only do evil deeds, but if your vengeance were near at hand, within your reach... you might do some pretty awful things.
And we know Mythal kept slaves, because of vallaslin--Abelas claims to be bound to her will if you ask about the price of the Well. She was never, ever 100% good, no matter how Solas may have idealized her or how many times she helped our protagonists.
Could I be totally wrong? Certainly. But the Veil is going to be removed, and it being a game, we already know we're going to be able to convince Solas to stop, or kill him. Whether we're simply too late or someone else is involved, I don't know, but if someone else is involved, the only one who's got the freedom of movement to intervene, that we know of, is Mythal and/or whomever happens to be bound to her through the Well.