So does everyone who frees Blackwall want the chance to free all the other people you have to judge? All the NPCs to you fight? Corypheus?
Why does Blackwall deserve to atone but not the others? Any of those other people could regret what they did given time like what Blackwall got.
I freed a lot of people that I judged and gave them second chances because - for me - it's far better to do that than it is to waste a life if someone can be redeemed. I ultimately sent Ser Ruth back to the Wardens to deal with what she had done, rather than making her a laughingstock (pointless) or sending her to the Deep Roads (a waste of a valuable individual). She clearly felt the weight of guilt and remorse, and she was going to live with it and atone for it by doing her duty as a Warden as far as I was concerned - it also doesn't hurt to have a voice of conscience to maybe help the other Wardens be more vigilant about their own actions.
Her case wasn't entirely dissimilar to Blackwall's, but A ) because I find the Joining to be too much of a risk for my LI and B ) I feel that the whole Warden thing has been kind of a crutch for him. I freed him to atone as himself with the Inquisition. Instead of throwing Alexius in jail or killing him, I gave him a chance by having him as an agent performing magical research. Considering that he had tried to personally kill me, I probably had more right than anyone to summarily execute him - but it would have been pointless, cruel, vindictive, and a waste of talent.
Vis-a-vis my conversation with Dorian - and I'm sure you don't really care about things like personal relationships or knowing about a man's character and nature, judging by your apparent stance on things - I learned that Alexius had actually been a good man prior to what happened to his wife and son. So I chose to spare him and show mercy. Got a problem with me being understanding that this was a good man who took the wrong path because he was truly desperate to save his son?
I maintained a fairly consistent position of showing forgiveness when possible and giving many people a chance to redeem themselves and even gave the Mayor of Crestwood a chance - and he was responsible for more deaths than Rainier was when he attacked the Callier entourage. The mayor was sent to the Wardens; he can atone with them and take his chances with the Joining, hopefully doing some good to make up for what he did to his own people.
You can either kill everyone who wrongs you and others by sticking to some rigid code of "justice" if you like, but if I see some kernel of goodness in a person or the potential for change (or have seen actual change vis-a-vis Blackwall), then I feel that that is something worth saving - and I will try to do that. People like Erimond can get the sword or a cell for all I care - maybe a cell, although I think that even with years and years of self-reflection he's probably the kind of guy who will never change.
P.S. Don't be ridiculous by bringing Corypheus up - he tried to destroy the world and is a threat to everyone. What he did is on a whole different level, and that particular fight is a matter of life and death for the entire world. And bringing up random enemies is equally silly - that there is death along the way is unavoidable, and the people you fight are agents of Corypheus or ruthless bandits, etc. If they want to lay down their arms, then great - I expect they pretty much won't, though. I think most of us should have enough common sense to recognize the difference between narcissistic sociopaths, megalomaniacal nutjobs, and those people who can be redeemed (well, I should hope so anyway - if a person isn't capable of recognizing the differences...yeah, well, I don't want to be that person).