Vis-a-vis Blackwall turning things around on you...I sort of get where that comes from. He had pretty much resolved himself to die, and then the Inquisitor kind of yanked him back from the brink of that. I think it was partially a matter of frustration and this emotional response because a large part of what he did was to protect the Inquisitor from having her reputation tainted by him.
And then what happens after all that? Depending on what you do, you either 1) calls in political favors and pulls strings for him by using Josephine's option, 2) maybe gets some people killed by using Cullen's option or 3) does something slightly shady by replacing him with someone already sentenced to death using Leliana's option.
In essence, I think he thinks he's corrupted the Inquisitor on some level and then sort of lashes out - it's certainly not deserved, and obviously, he's not in the best position to be making judgments about something like that, but I kind of get the response. It's like, really??? Did you really just say that after everything I did?
At any rate, if a character were perfect and did everything right, they wouldn't be nearly as interesting for me. It's like how I understood how Kaidan was pissy on Horizon and didn't just jump into Shepard's arms. Everyone was all upset about it, but I pointed out that it was a completely normal and human response for him to be upset.