Why wouldn't O'Brien, or anyone else, feel compassion for someone so thoroughly brainwashed by a malign entity that she could only free herself by committing suicide?
I get that a lot of people may not like Sarina Douglas for whatever reason. I just don't understand how people could read Control and not feel sorry for her. She is the person who made the decisions which led to the Federation being freed of Section 31 and Uraei, based on moral convictions that we can reasonably presume her readers share, and she suffered and died terribly because of it. There is no ambiguity about what happened to her not being terrible.