Perhaps....he took some of the anchor power from Quizzy in the way he took Mythal's power.
It's just confusing at first because he could've tried to take the anchor's power sooner...he could've approached the inquisitor immediately after his strength was restored.
But perhaps he was precognizant that the Inquisitor would eventually come to him, and his long-term strategy is such that he could be patient and await the encounter just so. Such that he expected the Inquisitor behind him as he stepped toward the Eluvian. It was all in his design.
He's "not a monster" and wanted the Inquisitor to have an arm a while longer. After all, they did need that arm and the anchor to do their part in stopping the Qunari, which Solas wanted as well.
I think it may be closer to the truth that people who said Mythal took over Solas were right. Maybe he's not entirely taken over, but maybe Mythal's will now drives him.
Although it's also likely he was under a geas the entire time--it would explain why his intent to do what he sees as his duty cannot be turned aside no matter what we do or say in Inquisition. And he comes very close, if you romance him... only he just can't stop.
Taking the Anchor's power back makes a certain amount of sense--with how it was doing massive magical explosions, it must have had a tremendous amount of magic in it. Dispelling it might not have been entirely successful. So absorbing that power might actually be wiser than setting it loose.
And yet, if Solas took our power, why is he in the Mythal role this time? Note he's in Mythal's position the whole way through. Why? Is it that he's the one losing something this time? If so, then... What? Himself? His connection to the Inquisitor?
There could be something to the magic colors. If Solas's anchor is green, shouldn't his magic be green? But he takes Mythal's power, and he's got a blue or violet glow now. Might be a silly observation, but it might also be relevant.
Something that might be important to understand what the parallel is about that Mythal expects Solas to come, and Solas expects the Inquisitor to come. Mythal knows what Solas wants, and Solas believes the Inquisitor wants answers (as well as rescue from the magic hand).
If Solas told us the truth about not wanting to cause unnecessary harm/death, then it implies that he believes Mythal's death was necessary to his goals. It also implies that he feels his goal is so important that it is worth any amount of death. Food for thought. If he had needed to kill the Inquisitor, the Inquisitor would be dead.
Solas viewed Felassan's death as necessary as well. Felassan had betrayed him, and was not going to help him. Therefore, Felassan had to die.
So why, knowing the Inquisitor will oppose him one way or another, is the Inquisitor's death not necessary? Or is it more that the Inquisitor's death would be too inconvenient for his plans (cause too much chaos etc) such that it's better not to kill the Inquisitor, at least not yet?
And this brings me back full circle to the question I posted as its own thread: Knowing he hates unnecessary death, knowing he prefers to be kind to people, knowing that if you friended or romanced him he sees the people of modern Thedas as people... what is so big about this goal of his, that he feels that the tremendous amount of death and destruction it's going to cause, is necessary? Because that seems to be the case.