The Mage Underground - "I have made this place a sanctum of healing and salvation! Why do you threat

Cullen was who said that. Still thinks, the mages need a Templar for constantly watching... eh...

What Anders did, did as an oppressed person, mage or not. And what happened later, the rebellion, justified him. But the war would be avoidable – if the Seekers/Templars just let them be independent – free. But they prevented the negotiation. The Seekers and the Templars started the war. For their wounded pride.

Tevinter's problem not the free mages, not even the Magocracy. It can be just like the nobility in Orlais. Tevinter's problem is the slavery and the acceptance of the human sacrifice. The truth is: not every mage is free in Tevinter. Many of them slaves or workers, as Calepernia and Varania.

Yes, it did. Do you want to continue?

Not only the future justified him. The past and the present too. Plus: he expected it will happen a revolution, so yes the future as well.

I know, you think, every revolution is evil. So hoping to start a revolution is evil. To pray to the Maker for freedom is the only acceptable act. I got it. 
The Maker answered
...

I said: just pray for it, perhaps, the freedom will come. If not, the Maker didn't want. Or perhaps didn't hear. So, pray for it louder! Or the Maker thinks, you're unworthy. Isn't?

It's already legal. The Chantry (the Templars) uses it legally.

Cassandra at first blames the Champion (Hawke) for the mage rebellion, that is until she interrogates Varric
at the end of DA2 she states
"Then Meredith provoked the Circle, she was to blame"
Varric replies with "or that damned idol was, or Anders...take your pick"
Cassandra: even so, had the champion hadn't been there...
Varric: it might never even gone that far

so yeah, Anders is partially to blame for the mage rebellion
but not entirely, tensions were already reaching a breaking point by the time that happened
The White Spire for example, mostly due to the research by Pharamond into a way to reverse/alter the Rite of Tranquility
Fiona was also a very vocal point of pushing for independence
there were many factors that decided that the mage rebellion would happen, Anders' actions were just a part of the puzzle

however, I still think killing Anders might be the right choice, as much as I like the fellow, I can't help but wonder...did he ever tell the truth, to anyone?
He outright blackmails Hawke (if they ask what he's planning)in enlisting their help to get the explosives inside the Chantry
(I still wish there was a way you could say you didn't know what he was planning to Sebastian, as well as apologizing to Anders before killing him if he was your friend)
When the Warden meets him at Vigil's keep, there are bodies of Templars around him, Anders claims the darkspawn killed them, since he's fighting the darkspawn when we meet him that's a safe bet. However the Templar who wishes to apprehend him states Anders is a murderer
so either Anders killed those Templars...or has killed someone before that event

Justice/Vengeance amplifies Anders desire for freedom and what he believes to be justice toward the Chantry and the Templars
he's seen abuse in his time in the Circle, and heard it from fellow mages. He hates the Circle, but killing innocent people is still wrong

I would have no strings but what the mages choose themselves. No compromises with society - it's not about the greater good, but about individual freedom.

The flipside is that what Anders did was still a bad thing, because the mages never wanted it or asked for it and he sacrificed their lives to his grandiose vision of how things should be. Dragon Age 2 was about Kirkwall and maybe Anders helped improve things for the majority of mages in the long run, but he threw Kirkwall's mages under a bus (then set it on fire).

actually, if you keep pressing Anders about his plan, he keeps dodging Hawke's questions

If it can help in the decision: what Sebastian did was blackmail, whatever's Hawke's reason to spare Anders' life (don't forget: that life will never same as were before...). And not like "if you would care about me, you would help me", but "if you don't be an executioner, I will destroy that city..." (not you, not your friend/love, but this city)... But Hawke's not responsible for Sebastian's sin..n (And send Cullen, he –and the army with Aveline's guards– will convince Sebastian to turn back.)

Meredith's very good at blackmail, an I found her manipulative, in her own way. And so clever parry, when she says: she mentioned Bethany because she's a good mage... congratulation, Knight Commander (my Hawke would kill her right there...) Like a Godfather...

I also understand Sebastian's first anger – of course, but after three years, in the middle of a world chaos?
(I love, that at this moment Sebastian's the real embodied vengeance... "I'll teach to your precious Anders, what the true justice is..." and after three years, he shows, what the true vengeance is... In fact, I like Sebastian's character, he's more interesting than a simple choir boy)

About the proverbs: we say: "the last drop in the glass" and the last straw... (I'm from Hungary)