My main problem with Anna's story is that it's the wrong story. THIS is the story that should have played out after she murdered Faison after he was handcuffed and "helpless". THIS is the story that should have played out after she murdered Dr. O in cold blood for being responsible for Robin's abduction. If Anna was going to kill anyone and be haunted by it because she is a "good guy" character (an interesting story, I agree) she needed the right set-up event. There were literally 2 there for the taking, and they ended up slapping a contrived ending to Duke's story in order to jump start this story arc.
Yes - Anna had the right to grieve Duke and want vengeance - but if the actions of Faison and Dr. O were not enough to push her over the edge, this shouldn't have done it either. But - Carlos was expendable, and Faison and Dr. O are not - which makes the story feel contrived to me. I think the actual scenes are well written and well directed, and I think FH is acting the HELL out it - the best "material" she's had in years. But it still rings false to me. Too convenient - more of a way to get Anna off screen for a while and re-set Slone's character.
I've always thought Anna was capable of murdering someone in the right circumstances - Faison, Olivia Jerome, Grant Putnam, and now Dr. O. They all needed killing. Carlos needed arresting and prosecuting and throwing away the key.