It been a LONG time since I've play dragon age. I was so psych for the DA:keep So I've re-Bought DA:O ultimate edition and DA:2 and all of it's DLC so I can make my perfect "canon" and will be ready for DA:I! But then as I was approach the end of DA:2 I remember something I shouldn't have forgotten.........

Anders.... My best friend and vice versa, just killed a whole building of innocents people! I have both killed him and let him go. But as DA:I approach and the release of the Keep gets closer I'm torn part by what decision I should stand by. People would argue it was necessary and justified, But It's like Aveline said " Belief is no excuse. Sincerity does justify this, " Isabella and Anders had a party banter about justice....
Anders: There is justice in the world.
Isabela: Is there? You want to free the mages. Let's say you do, but to get there, you kill a bunch of innocent people.
Isabela: What about them? Don't they then deserve justice?
Anders: Yes.
Don't the people in the Chantry, who were just praying to the Maker deserved justice too?.......... Well that's the end of that. Could you guys help by making me make a final decision?
Don't see how it's justice to kill innocent people who were never involved in the whole decision to treat mages as non-people worthy of humane treatment. It's a way overboard reaction, designed specifically to incite riot and mayhem. It served no other purpose. If injustices did exist, which I believe we can all agree did (tranquil of mages for ridiculous offences that had nothing to do with potential abominations, annulment of all mages, imprisonment without any rights whatsoever), there had to be a better way of addressing them short of forcing all mages and templars to go at each others' throats without restriction.
But that's essentially what Anders did. By destroying the chantry and all who were inside, he declared open war on the templars and the chantry and involved all surviving mages, whether they wanted to be a part of it or not. He deserved to die because allowing him to live only condoned his actions.
Sebastian, on the other hand, was struggling to put right what had been done to his family. He didn't decide to blow up an entire city just to kill those that had harmed him. He went after those who had wronged him, specifically. Saint? Hardly. Entitled? Yes. He felt privileged, entitled and robbed of his legacy and birthright. If you'd been in the same situation, I hardly think you'd be more humble about any of it.
Leliana had been manipulated so much during her life, she became convinced that it was the only way. That's what was refreshing in DAI. You have an opportunity to show her that things can be resolved differently. She begins to think of her own past actions and beliefs differently. She's a work in progress throughout the Dragon Age series.