Maybe.. Or deciding to leave the party with Susan, and leaving Michael to die, and one of the Swords to be destroyed. Harry tossed the idea around in his head before he decided to back Michael. He was given the choice. Who is your guest? Michael, or Susan? If he chose Susan he could have walked right out. He chose to stay, fight, and lose Susan (basically), and start a war. Or he could have left with Susan, and a KoTC would have been slain, Excalibur destroyed, the Red Court war would have been postponed..
Personally at the moment I'm leaning towards Harry leaving with Susan.
This choice seems like a good one to me, but it doesn't happen near the end of the book; and Jim stated the choice Harry made happened near the end of Grave Peril. So, if you think that Harry saying "I love you" to Susan is too easy; or actually it wasn't a choice at all, it was a spontaneous utterance that Harry didn't think about or weigh the consequences of saying something else, there is one other choice Harry made that happens much nearer to the end of the book. Harry could have killed Thomas; or actually I think Michael would have done it, when Thomas was trying to get Harry's help. In fact, Michael was pushing pretty hard for the "let's have done with it and just kill the vampire" option. For a moment it looked like Harry might go along with that option.
Think of the ramifications of making that choice. It would have poisoned Harry's friendship with Michael, but only after Harry discovered the truth of what he had done. Thomas had information on where Susan and Justine had been taken. So without that knowledge to go on Susan would have been turned and Justine would have died. Harry would have realized he had made a mistake; and I think Michael too, if they had killed Thomas and then found out he had brought Amorachius to them. In this scenario it would have taken some time for Harry to learn the truth. The other possibility is having no real clues to go on Harry would have tried the break-in to Bianca's place anyway. He would have ended up in same cell but while talking to Justine he would have told her that Thomas was dead and she would have told Harry that Thomas was his brother. Now in this scenario Harry is still dealing with an infected Susan about go full vamp on him. I'm not sure how this plays out. Also, Harry still had to get his mojo back from Kravos' shade and he needed Susan help to pull it off.
Those issues aside, I think this alternative choice pushes Harry to the darkside as much or more than any other he could have made.