It's amazing how you can write so much, and yet address my points so little. Aside from the fallacies involved with demanding fiat justitia ruat caelum lest your justice system be meaningless, there's not much you actually raised here about whether Blackwall submitting to the justice system is justice or compatible with his own view of atonement.
Allowing himself to executed was an alternative to his better option being recruited into the Wardens. Blackwall may have been willing to die and would have if it meant saving his former comrade, but since that person was saved and the Inquisition was able to send him to the Wardens then Blackwall was able to receive a form of justice and atonement that he most obviously preferred then execution. A later point of yours talks about how our proprieties are obviously not straight if we let Blackwall live, that were playing character favoritism which can be associated with blatant nepotism. First off, many people in this thread have said why this is not the case, with logical reasons as to why they believe letting Blackwall live is a pragmatic, necessary action. Second, your coming at a lot of people with an accusatory moral high ground that infers that no matter the circumstances your PC will not allow personal entanglements to determine how you judge people. Let's just assume for a second that objectivity is actually a genuine thing that exists and that no small part of you has any sympathy or bias for any character you've meet in the DA universe. Are you telling me that;
When Sten tells he murdered children that you recruit him. Content be damned.
When Leliana tells you she was a Orlesian bard that you accused her for prior acts of espionage and assassination and kick her out of the party.
When Morrigan tells you that she was part of culpable murder and the flat murder of templars that you didn't kick her out as well.
When Shale a creature with no legal standing tell you she gleefully killed a man that you didn't kick her stony ass to the curve.
That Merrill, Anders and Bethany were given to the legal authority for handling the mages, the templars.
What about Isabela, the pirate, Fenris the runaway slave, Sera the thief, Cole the mage murderer and demon, Iron Bull the spy.
You're telling that if the games ever gave you the opportunity to make this characters face justice for their crimes you'd treat with the same level of prejudice that the justice system of Thedas as written calls for? Archdemon slaying, Kirwall saving, Corypheus defeating needs be damned.
If so, then you my friend are committed to your legal standards.