This is like Loghain all over again, except worse since Blackwall was actually trying to make up for his past sins. Something very few have the balls to dedicate their lives to doing. He could have very easily gone about his business and lived without ever giving a damn about what he did, but instead he wasted his time going around helping people as a warden, then tagging around your ragtag group.
And yet, people still wish him dead or away. I don't get it. It's like the people who kill Paarthurnax on Skyrim. No sense for redemption. How do you punish a man who's already punished himself more than you ever could. He wanted to die. What does that tell you.
Man murders your entire family, including children, in cold blood, goes into hiding. He joins.. let's say the red cross and dedicates his life to charity. Turns up 20 years later by his own free will.
You: Oh well, all is forgiven, no punishment for you?
He's a murderer and the fact that he turns himself in means that he knows he deserves his punishment. Since his crimes were performed in Orlais, I am perfectly content with them handling his trial. I might gave given a good word for him in any eventual trial, but that's not an option. It's not like the life of a hermit "Grey Warden" seems to be much worse than what everyone else is dealing with.
Besides, do you think that lieutenant was the first of his men to be captured and executed in his name? He's indirectly the murderer of even more than just that family by not turning himself in sooner.