The backlash among some fans to Blackwall is amusing considering...
Sten is guilty of pretty much the same crime. Like Blackwall he murdered a family, and was sitting in a cage for it when you first find him.
Zevran is an assassin. He murders for coin.
Leliana was a bard. The role of bards in Orlesian society is as spies and assassins. The man difference between them and the Crows is that they are a little less mercenary, and work for noble patrons or the Orlesian crown. Still as agents they are among the primary tools of Orlesian aristocrats playing the Great Game, and do lots of awful stuff.
Isabela is a pirate, which is nothing more than a seagoing bandit. She murders for coin. In her backstory she even once dumped a load of slaves into the ocean to lighten her ship, so she could flee a pursuing warship.
Despite DA protagonists consorting with far worse people than Blackwall, I don't recall as many people clamoring for the protagonist to lop off those other characters heads. The real 'crime' people want Blackwall punished for is that he lied to the Inquisitor. Maker, we can't have that!
There's also Velanna, who slaughtered a load of humans because she thought they'd abducted her sister. She shows a little remorse in banters with Justice, but not nearly to the same extent as Blackwall. Granted, she's not a popular character, but that tends to be due to her personality, not her homicidal rampage.
We don't know much about Morrigan's past actions, but she supports every 'evil' act in the game, from massacring a Dalish clan, to abandoning Redcliffe, threatening Revered Mothers and kicking puppies.
Fenris murdered a bunch of Fog Warriors after they'd taken him in. Sure, he was suffering from the trauma/brainwashing of his enslavement, but if people are going to throw stones...
Not to mention Loghain, though opinions are pretty divided on him too.
There are definitely a lot of companions with bloody pasts. Seems weird that Blackwall gets singled out simply because he lied about it, disregarding that he does more than all the others to make amends for his past actions. And seems to spend a lot of time regretting them.