See, I never liked that part. It wasn't his first cold blooded killing. He killed Grevanes drummer (for the life of me I can't remember his name).
Sorry - I should have been more specific. The text I quote (from Proven Guilty) is about Harry's murder of the former nickelhead that Harry had tagged as Liver Spots. Liver Spots was in bad shape. He was aging fast, had just had the stuffing kicked out of him, and if left alone would have probably died soon. Even if he had recovered from that beating he maybe a month, if that.
But Harry didn't leave him alone. He killed Liver Spots, murdering a defenseless, helpless, old man. It was an evil old man, one that wanted to form another partnership with one of the Fallen, but helpless and on the way out.
Harry gave into his rage and anger. He was probably justified - this time. But beyond the feelings about killing that time, Harry had to wonder if he would do that again. Wonder if he would be justified next time. Wonder about what sort of man he was becoming. Wonder about what other bad calls he would make.
And then there were Harry's insecurities. For most of his adult life the people who should have been his peers and mentors (the White Council) had been watching him for signs that he was evil - tainted by the killing of his first master. Looking back on that murder, Harry had to wonder if his detractors had been right. Was he going to become a Dark Wizard or Out Of Control Mad Dog Warlock? Killing that man had felt like the right thing to do - so how could Harry even trust his feelings in the future?
If he had never have done that... I can see Harry killing Slate and telling himself that it was a mercy killing (which it was) and that he was only doing it for his daughter (which he was) and otherwise excusing and rationalising the act. But I can't see him killing Susan without first having killed Liver Spots. She was someone he loved. As he killed Susan, Harry hated himself for doing it - but he did it. He knew that he had to kill her, that it was the only way to save their daughter, but without that first cold blooded killing under his belt I think that Harry would have hesitated for too long and lost his chance.
And that's why I think the stain from the murder is slowly spreading, making it far worse than touching the coin.
Richard