@Dai Grepher: You won't get much argument from me. I think Solas is a bit of a mess, and I think a lot of people find patterns of logic where none actually exist where Solas is concerned.
You touch on a point that drives me nuts about Tresspasser... an Inquisitor that promises to destroy Solas should have been destroyed. Right then and there Solas should have looked down on him pityingly and said: "I know you would try." And then proceed to have the mark consume the Inquisitor.
People who want to see Solas as some deep, misunderstood figure of immense wisdom will look at this and say: "Oh, obviously he's showing compassion here." When, in reality... it's more likely just Bioware plot armor and an attempt to avoid player outrage.
An Inquisitor that wants to redeem Solas should be left alive. Redemption isn't going to happen (except for perhaps self-annihilation) - but Solas would have no real reason to kill an eager pet.
We already know that this elf wanted to get his magical item working... so he gave it to a Darkspawn Magister knowing full well the ritual that would be involved because he expects the Darkspawn Magister to "die in the explosion".
The Dalish tales of Solas as the Dread Wolf paint him as a villain... so him killing Felassan is not at all out of character, unless you want to believe the Dalish tales are all wrong. They could very well be. We know the Dalish are more wrong about elven history than right.
However, when you look at Solas as a whole... you get tales of his predatory nature in the past AND factual data in the present... which suggests that there is more a chance that the tales are true than they are false. Had Solas spent his time during Inquisition say... playing Gandhi to the elves of Thedas... then his nature as the "Dread Wolf" would be far more suspect.
As it is, I believe all evidence points to the fact that Solas has always been... then and now... a being devoid of empathy and prideful enough to be megalomaniacal. To me it makes perfect sense that he kills a someone that displeases him (by screwing up "The Plan")
This is why I believe the story he feeds the PC is totally false and a manipulation of the events to try to get you to agree with him.