Solas betrayed the "Forgotten Ones", and the "Old Gods" both, and this caused the entire collapse of Elven society. Rather than trying to fix things, he decided to take a really long nap and play with demons. After his nap, he sets forth in motion a set of events that would collapse human society into oblivion. When that fails, he sucks the soul out of his old "friend". This guy is a massive betrayer.
Even his parting message to you, if you have gained his approval, is "No matter what happens in the future, know I have the utmost respect for you". To me, that says he's probably going to betray you. Made some good Templar/Warden allies? Well, "Solas Greatly Disapproves" because you gained in power. Allying with the mages, however, is an opportunity for future chaos, so that's most welcome. Helping the little people shows Solas you're a chump, so he likes that too. Listening to his nonsense and lies? Ahh, he has you around his finger.
Words are one thing, but actions are quite another. Should be some option to turn this jerkoff tranquil, and save everyone some future headaches.