But it wasn't just anger, not... simply anger, at least. It was injustice, in that case.
Preserving the balance of Order and Chaos that makes free will possible requires death. Sometimes the only people who have to die are unarguably Bad Guys. Sometimes, though, innocents have to die so others have the chance to live. It's the trolley problem- the necessary consequences of absolute pragmatism: the end justifying a means that is in itself always morally Good, morally Right.
[following train allegory]
First off, I do not think that there is any inevitable necessity to kill (except for self-defense, I guess). A lifetime of being Batman just forces me to disagree. More than that even I am convinced that killing does not solve problems. Killing eliminates a creature, never a thought. But a creature is easily substituted by another to continue its atrocities. So the killing will never end, but neither will the atrocities. As long as the thought endures, the killing is useless.
Maybe I can put up with a very particular situation, very detailled and sketched out, that allows for "killing was necessary and good". But definitely I will not agree with a generalised "killing is necessary and good, if...". Forgive me, but this is too much to ask of me.
Second, Solas killing the mages has nothing to do with that train scenario. There was no need to kill, however one defines "need", it was pure bloodlust. There would have been a thousand ways to make them pay. There were no negative consequences to letting them live, no people to be saved, no orphans to be rescued. And the "balance of order and chaos" was never threatened by those few ignorant mages.
Third, on a more general note, I do not agree that the end always justifies the means. More than once such thinking led to dictatorships. Who decides when a cause is just? Castro overthrowing dictator Batista and taking over control surely was considered a just cause by many. And for a while, at least. When he tried to make his idea of right and wrong endure by installing a new dictatorship, did those means justify the end? And if so, did they lead to an improvement of the situation? Whatever my personal opinion here, is there ONE right answer? Just as one example, I will avoid further ranting.
Forth, just because I am halfway through to hell already anyway :-), about the train: who is the one person and who are the 5? Is the one person the one who will change history, the one with very important knowledge, the one Mother Teresa? And the 5 are a group of notoric drunks who like to beat up gays? Are the 5 a group of human rights activists while the one is the leader of a KKK group? What I am trying to say is that this question is meant to cheaply provoke thought, but it does not really give much away in terms of testing your character. Would I let 5 or 10 or 15 people die for a single person? "It depends." Yes, if the one seems "worth more". If I know the one and think very highly of him/her and I do not know the others, would I save the one? Probably. Because I can assess what good s/he can do while the others could be a bunch of bad apples.
Now, what am I? Good or bad? I cannot tell.