It's ... complicated.
I don't like Sera. Not the way I love Vivienne or adore Solas or positively absolutely cannot live without Dorian/Varric. I find her personality abrasive, her mannerisms disgusting, and her opinions shallow and simple-minded.
But at the same time she's a very heartbreaking character and has some surprisingly perceptive lines that are really important for your PC to hear, esp as an Inquis. There is one line, I think after the Winter Palace, where she tells you that it doesn't necessarily matter who's in charge - changing one person at the top won't change anything for the people who are truly/continuously suffering. She's shortsighted insofar as she has not real plan for change or wish to create a society where her actions become obsolete/unnecessary, but she does help highlight the fact that systemic, bottom-up change is necessary for truly meaningful change to occur in Thedas. Take what she says in partnership with Dorian's perspectives on slavery. I don't agree with Dorian of course (Southern slavery apologism at its best really) but he has a point when it comes to the lifestyle of the people at the bottom of Ferelden/Orlesian society. As long as there are distinctive classes with no genuine mobility between them, the lives of the people at the bottom of society are going to continue the way it is regardless of what kind of "society" it is or who (mages/Chantry/royalty/Free Marcher noble) is in charge.
Sera isn't supposed to be viewed as a Robin Hood/freedom fighter stand in type character (also remember, Robin Hood was originally from the nobility, in fact, most Robin Hood-esque characters across cultures are actually individuals of noble origin who have a sudden "aha" moment and turn their backs on their own privileges) - she's supposed to be a victim. So I think the problem when people really hate her as opposed to just disliking or not getting along with her is that they're not viewing her within the right context (i.e. the guy in the video someone linked on the first page) - she's a victim who sees herself as a freedom fighter not an actual Robin Hood trope. We don't have to like or agree with Sera, but I think its important to listen to her and understand how she exemplifies the perspective and experiences of people we have limited interactions with in terms of our other companions.
She's also an interesting character if you consider her rejection of elfy things - because she dismisses the Dalish for being self-victimizing and laughs at them for erroneously construing much of their history, but at the same time, she still self-victimizes and utilizes much of the Dalish-human logic in her arguments/actions towards nobles. I do wish BW had given her a more meaningful growth arc and I didn't really take her anywhere because like I said, I don't like her as a person, but I do think she brings value to the cast and storyline.
(Don't fight me on Viv though I love Viv and will defend her and her bitchiness to the end of time. Viv is all my human mage Inquis wanted to be. Viv is all I want to be. Viv is life. Viv is aspirational goals. Viv is perfection.)
The order does matter, at least in fictional entertainment. A good person or rather, a character presented in the story from the beginning as a good person but grows progressively worse and more horrible (like Loghain) comes off as very repugnant. A bad person that grows progressively better by developing stronger morals and integrity comes off as very likeable and endearing. Its the anti-hero archetype. Lots of people find the bitter, world-weary anti-hero types to be more popular and endearing compared to "heroes" who have never done any wrong and are bit too idealistic with a hint of naivety.
And your exaggerating Thom Rainier's crime. He did not kill a family without a purpose, he did so for money. He did not know the nobleman he was hired to kill was accompanied by his family. He also did not personally kill the nobleman and his family, he ordered his men to do it. Rainier wasn't even present at the scene of the massacre. If anything, his men are more of monsters than him because they could have stopped themselves from killing any further after finishing off the nobleman. Rainier's sin wasn't the direct murder of the family, his sin was ordering it and then letting his men take the fall while he ran away like a coward. Yeah, this does not make Rainier/Blackwall any better but I do wish that you do not exaggerate the nature of the events of his past just to strengthen your argument.
I'm well aware that he's a base breaker in the fanbase. I do like to point out that even in-game, he is hated by quite a lot of people in Skyhold, at least based on his explanations (I would have preferred a cutscene showing the Skyhold people refusing to sit near him or soldiers refusing to train under him). He's lost his friendship with Cassandra, Cullen hates him but at least respects him, mocked by Vivienne and while still friends with Iron Bull, one banter (if the Chargers were sacrificed) has Blackwall calling Bull out for leaving the Chargers for dead only for Bull to point out how its different from "Rainier's" situation; Bull did not abandon the Chargers as he stayed and watch in horror at their demise, and Bull honoured their memory by giving them a funeral. Blackwall merely ending the banter with saying that he would never listen to Bull if he ordered Blackwall to hold down a hill.
Really though, I think its obvious that Blackwall was meant to have his fair share of haters, in-game and in real life just to drive home the theme that the path of redemption is an excruciatingly difficult path but in all honesty, if he was instantly redeemed and still liked by everyone, I would have definitely hated his character. The difference between him and Sera is that while Blackwall will be hated for his actions in the past, Sera will be hated for her attitude and the opinions she holds in the present.
I like your analysis of the reactions to Blackwall (also remember Solas who used to be friends with him losing all respect for him) but I disagree with your diminishing of his crime. Would you argue then, that the Nazis who carried out the actions in the extermination camps during the Holocaust were of greater culpability than the senior officers/Hitler who gave the orders? The "just following orders" defense is a weak one, I agree - it's how many Japanese war criminals were never prosecuted for their actions in China/Korea/SE Asia - and its quite horrifying, but I also believe that a greater measure of responsibility is always held by those who gave the orders or failed to properly restrain those who were within their command. It's the common saying "the buck stops here" - a commanding officer is responsible for the actions of his subordinates even if they weren't explicitly ordained by him, not to mention actions that stemmed from his orders.
Moreover, it's emphasized by Blackwall himself time and time again as well as the lore that his soldiers had complete and utter faith and loyalty to him and would have followed his orders to march straight down to hell itself. Think about it in terms of the templars - many of them do terrible things throughout the game series, but only out of blind loyalty to the Order and an inability to think for themselves. Are they culpable? Yes. But more culpable are those who misused their trust and abused the power they held.
Finally, we know through banter between Blackwall and Cole that he was indeed present at the time of the massacre - he heard the song the children were singing in the carriage before he gave the order to attack. (Rests her case)
I don't hate Blackwall, he's a genuinely decent person (and such a good MT I couldn't kill him if he'd murdered Andraste herself) after everything that has happened, but really. He's very guilty and he's right when he says that he deserves to die. Which is an interesting perspective on the death penalty in general - Blackwall was lucky that he was able to escape immediate consequences for his actions and have the chance to reform and redeem himself and become a moral person - how many people aren't so lucky for lesser offenses? (Not my in game problem though~ I never executed anyone fufufu redemption for all!)