You recruit plenty of reprehensible people across BioWare games
Sten
Jack
Miranda
Morrigan
Anders
Thane
Zevran
Lilliana
murders and assassins all
for some reason Blackwall seems to be getting a little more vitriol than most
I think people are madder that Blackwall lied to them about being a warden than about what he actually did
That being said I'm not sure I see any reason we shouldn't be able to execute him
Edit:
by Han
Yes, but the difference is that Blackwall lied about who he was and withheld information about what he did.
Thane, Jack, Sten, and Zevran were both up front right away about the fact that they had killed people. Sten may have not told you right away why he was in the cage, but he did tell you he deserved to be there (implying he did something bad). Morrigan never hid behind a mask, and she also never killed innocent people or impersonated another. Leliana may have taken awhile to tell you who she was, but she did say right away that she had a colorful past, and she never impersonated someone else to escape from it. Anders never hid the fact that he was an abomination, and potentially dangerous because of it, either.
Blackwall is not only a murderer (one that is responsible for the deaths of even children), but he let his own men suffer from it, and then ran away and impersonated another to escape his crime. Impersonated a man that was honorable, which some could say stains his death. He lies about all of these things to you the entire time, even when it becomes blatantly obvious that he knows nothing about Grey Wardens during the Adament arc. Not to mention, he hides behind his fake persona and acts all uptight and honorable to other party members about their actions, which he really has no right to do, considering his past (such as condemning Iron Bull if you let the Chargers die during his personal quest, despite the fact that Blackwall basically did the same thing to his men).