I think that Blackwall is the perfect example of how people perceive a man in a diametrically opposed way depending on how they focus on the good or on the bad.
I personally focus on the good, and Rainier/Blackwall did a lot of good.
I read a lot of times people belittling what he did as a warden - because yes, he lived, acted, fought and suffered as a warden for many years, taint or no taint.
'I don't care if he killed some darkspawn... saving some refugees is not enough...'
I everyday hear calling 'heroes' people who saved one single human life. He saved many.
Many people seem to forget that most wardens were criminals, and it doesn’t matter what they did when they join the Order because joining the wardens means erase a man's past. The only difference with Blackwall is that we know all the details of what he did, while ‘having been a criminal’ is vague.
I never heard anyone asking to personally execute Cullen for locking all the mages, children included, in the tower of magi with the abominations in DAO, nor to personally execute Zevran for being an unrepentant assassin, nor wanting justice for his victims.
Just to make one of the many possible examples we, as part of the Inquisition, accept to kill all the apostates in Witchwood. I am sure they were no monsters, just people who, as children, were stolen from their parents in order to be locked inside a tower, then suffered imprisonment.
Now, for the first time in their lives, they are free and are fighting to keep their freedom. To their eyes you, the Inquisitor, are nothing but one more pain in the ass. I'm reasonably sure there were also children or teen-agers among them but I'm sure nobody thought about it before entering the cave to exterminate them all.
Heroes are made, not born. Who knows what the 'real' Blackwall did before he became a Grey Warden, perhaps he murdered dozens of people, children included. Does it matter?
He became a hero and gained the silver wing of valor. If he ever met that kind of 'justice' instead of being recruited in the wardens the men he saved would be dead as well, and the world had lost a potential hero and all the good he could have done.
Callier for his part was not a poor victim. I think most of Blackwall's story is about his commoner background. He is not noble by birth, he's just a soldier that earned a better position with hard work and liked money [not so far away than any of us]. He accepted to kill a single man to get more coins, but happened that such man was travelling with his whole family. Obviously, the powerful people that hired him - and knew - did not say a word for helping him.
Blackwall is a character about how high classes can mess things up and get better results than those who are from lower classes, those who have to live the daily ****, inspired by the idealization of the heroes, understanding that they never could be one.
Rainier has a lot of self-esteem issues based on class, and also a lot of bias and hatred towards powerful people who don't give a damn about those who are under them.
When you first meet Krem he says about The Iron Bull: "He’s professional, we accept contracts with whoever makes the first real offer […] We’re loyal, we’re tough, and we don’t break contracts. Ask around Val Royeaux, we’ve got references."
This means that what Blackwall/Rainier did only once in his life, i.e. accepting a hire job to kill an Orlesian Grand-Gamer, was in fact Iron Bull’s daily bread and butter.
The Iron Bull and his chargers made a living out of killing shems, as Skinner will later say in Skyhold - and she seems to enjoy it a great deal too.
The Iron Bull accepted every contract if the offer was good enough. Krem: "You’re the first time he’s gone out of his way to pick a side."
This means that The Iron Bull wants to stop being a mere mercenary and wants to fight for the greater good…. BUT, he doesn’t join for free like Blackwall, he wants to be paid good money for it.
Rainier was not a professional criminal, he was a well respected, honest military captain who accepted only one contract as mercenary that ended in a massacre of an entire family. He lied to his men and didn't stop them when he heard the young voices, ok. All I can say is that he was a very clumsy and inexperienced mercenary, and he probably didn't tell his men because he was ashamed of what he was doing. He did wrong, but he's just a man.
All our companions have done very questionable things in their lives, and some still do (see Leliana). Does it matter? No, because they all are in the Inquisition to do good, to help save the ****** world, and if we start executing all the companions or advisors who did wrong or caused deaths we'd finish all our runs in solitude.