If he's the one who trapped them he may feel responsible for freeing them. His goals may be more of the metaphysical, he does fangirl the magic of Arlathan pretty hard. There are a lot of things he could be after.
The 'gods' were jerks but Solas comes across preferring the horrible things of his time to the horrible things of the present. Has his head up a thousand years ago, as Sera says.
Should be interesting to see how that goes. I mean seriously I love Solas but his logic is so flawed there. That's partially why he's such a compelling character for me though. He screws up, he tries to help correct his mistakes with Corypheus (he really sucks at making decisions), then sometimes he really doesn't and can more than a little bit of an ass. He doesn't even try to consider that his choices might be bad, even if you convince him the Dalish might not be as bad as he thinks. And even then that's not a "hey I changed my mind, babe, you've persuaded me. Maybe I'll stop talking crap about your people." You never persuade him fully on anything.
Worse, my Lavellan didn't get an opportunity to at least get a better explanation out of him. Again, need DLC so Lavellan can call him an idiot and then make out with him and/or try to stop him. I can do all three right?
Ugh I have problems. I didn't expect to read so far into this romance haha, but here we are.