Idk how mentioning Orlais being corrupt is somehow a defense of Blackwall. That's like saying every criminal should go free because their country is corrupt.
The reason people are mentioning it is because some people have implied that law and justice/doing what is right are one and the same, and an awful lot of them seem to claim that anyone who chooses not to follow the letter of the law is somehow wrong for allowing their compassion to supersede the law. It's perfectly valid to point out that the law and Orlesian society in general aren't particularly just and that they are, in fact, flawed at the core. It is pretty hypocritical of those law-and-order types to say to others that they're wrong for choosing to show mercy and to constantly throw "the law is always right" when that "law" happens to be unjust in the first place.
And no one ever demanded that you or anyone else like him. If people can't forgive him/whatever, that's cool. The issue is that those hardcore "you have to kill him because the law says he should hang" types tend to be the ones who demean others for making perfectly valid choices and then proceed to moralize to no end about what terrible people we are for showing mercy, all while claiming that those of us who aren't in favor of the hangman's noose are morally inferior, have a warped sense of justice, and/or are blithering idiots - which is clearly not the case.
What's particularly grating about that is how a lot of those people who insult us, etc. are perfectly fine with their past companions having committed murder/killed people and will excuse it away because they liked Zevran, Leliana, etc., but in this one case, they're not okay with it....because....who knows? Maybe they don't like feeling like their Inquisitor wasn't omniscient and didn't know everything? Maybe they don't like that Rainier isn't falling all over himself with gratitude if spared? Maybe it's that they have to deal with an ugly truth about killing and murder, instead of having murder be some funny anecdote like one of Zevran's stories or a tale told by a pretty bard like Leliana?
Really, I don't care what someone does in their own game or if they want to kill everyone under the sun in the name of their brand of "justice," but they shouldn't insult everyone who doesn't fall in lockstep with their beliefs and act morally superior when they've probably quite happily accepted the flaws of past companions who have been hired killers, etc. And none of us who ever spared him said he was a good man when he was Rainier of six years ago (he wasn't) - we've only pointed out that he has changed and become a good man in the intervening years to the point that he is willing to die to make up for what he did. If we choose to spare him for whatever reason, that's our right to do so (preferably without constantly dealing with insults or attacks on our character).