Jean Rostand said, "Kill one man, and you are a murderer. Kill millions, and you are a conqueror. Kill all, and you are a god." A popular dilution of this is "Kill a few people, and you're called a murderer. Kill countless people, and you're called a hero." There's a specificity to Blackwall's crimes. We can assign a number to the people who died as a result of his actions. And some of them were children, who we irrationally prefer to believe are only ever bystanders in the ugly games that are waged in the name of power.
Apathy decreases when exposed to details and easy-to-picture numbers. It's why charities use pictures of individual children in campaigns to combat hunger and tell you how many mouths will be fed by your donation. It's also one reason why Blackwall doesn't get a pass in the same way as say, Leliana or Zevran does. Their crimes may be equivalent (and they are certainly more numerous) but because the details are vague, we aren't so upset by them. JMO, of course.
I'm a little late but...
The version I know goes like this:
Kill one man and you are an assassin.
Kill ten men and you are a monster.
Kill a hundred and you are a hero.
Kill a thousand and you are a conqueror.
It's quite interesting to see how the perception of something as awful as taking another person's life changes based on the amount and the context.
Back in the day it made me see characters like Alexander Magnus, Caesar, and other historical figures in quite a different light( I was a teenager, I didn't think too much about such things at the time ).
I have to say that I didn't have that much of a reaction when I learnt about Blackwall's past. Actually it made me remember something that happens in my all time favourite comic: Berserk by Kentarou Miura.
In this comic the main character, Guts, is asked to assassinate none other than the King's brother. The one giving him the job is Griffith the leader of the mercenary band he is part of. Griffith ask this after someone tried to kill him during a hunting day with the King and finding some info that points toward the King's brother.
Guts sneaks into the castle and kills the man, but before he leaves someone spots him from the doorway. Without seeing the person in question Guts attacks... and when is sword has already gone through the person's chest he realizes that the person in question is none other that the son of the person he just killed. Guts is horrified by what he has done, the kid was no more than 12. Guts is distressed and almost gets caught.
Meanwhile Griffith is at a ball back in the castle. Then the news about the murder of the King's brother and nephew reach the party and... Griffith makes this CREEPY.AS.HELL face, I mean pleased doesn't even cut it, it is almost a delighted face at the news that things are going according to his plans.
During this part of the series we see Griffith socially climb thanks to his war achievements and the removal of anyone opposing him in court without a hint pointing towards him.
As he later says: "This is war. There is no spectator seat on a battlefield"
I never thought Guts was a bad man because of that event, same with Blackwall. They just happen to live by the sword so when **** happens it gets really shitty, but we all mess up things at one point or another, such is life.
PD: Oops, what a rant!