"One day I hope God will forgive me for giving birth to the idea that came next. Because I never will. I knew how angry she was. I knew how afraid she was. Her child was about to die only inches beyond her reach, and what I did to her was as good as murder".....
Sounds like he made a deliberate decision here. Hey could have thought of something else, but he chose this.
Or once again it is Harry's conscience beating himself up overly, even though he did all he could at the time to save both her and their daughter. They were losing the fight, Maggie was about to die, so what did Harry do? Told Susan the truth about Martin. He may call that murder, but it wasn't, Susan still had a choice whether or not to tear Martin's throat out, she knew it would complete her change. Heck it may have happened anyway the moment Maggie's throat was cut, and there wouldn't have been anyone or anything to stop her turning.
Susan's final words..
"Help me please, save her please.." Um, at that point killing the Red King wasn't going to happen, in spite of all his efforts, their daughter's throat was about to be cut.... Once that happened, Harry was dead, Maggie was dead, Eb was dead, and Susan would be a vamp.. Another way to save their daughter? I don't think so, Holy Knights, powerful Fae, Odin, the Black Staff and others of the Grey Council fighting to stop it from happening, and they were still losing, Maggie's throat was still about to be cut by the Red King.. Also hate to get technical here, but when Harry did cut Susan's throat, she was no longer Susan, she was the youngest Red Court Vamp. So he didn't murder Susan, he murdered the youngest Red Court Vamp, if she was still Susan the spell reversal wouldn't have worked. So like we see in Christmas Eve., Harry beats himself up with woulda, shoulda, coulda, when in truth he did the best he could and it wasn't enough. Even though it was the only way to save little Maggie, and her mother was no longer a human at the point that he killed her, still hurts and will always hurt, so he beats himself up.. But did he really have another option? No, is the answer.