I'm not sure I follow this line of logic.
That might be because I'm poking holes at someone elses "it was justified" theory. If shooting Cassius was justified then killing Marcone is more than justified. Personally, I see Harry having options with Cassius and deciding to kill him - to murder Cassius in cold blood. At the time Harry sees the killing as the right thing to do, but later he has problems dealing with the fact that he killed in cold blood.
You seem to be assuming that everything that's dangerous is also a threat, and should be dealt with using a pre-emptive strike.
No, just pointing out that since day one in Storm Front, Marcone has been a bigger threat than Cassius was at the moment that Harry killed him. If people feel that Harry killing Cassius was the right thing to do then Marcone should have died as well.
Instead, Harry repeatedly risked his life to save Marcone in Fool Moon. When Harry got to the estate and saw the downed wolves Harry knew that Marcone now saw Harry as a threat and may have already sent out countless hitmen, but Harry still did what he had to in order to save Marcone. The "smart thing" to do then would have been to try to kill Marcone and the FBI agents rather than to try to take people alive. But Harry didn't do that then because Harry is not a cold blooded killer.
At least he wasn't until he murdered Cassius.
And that mundane murder has affected him as much (if not more) than the supernatural darkness that Harry has encountered (which is why it is on topic). Because Harry knew that he had a choice and that he decided to kill a fellow human being.
And the fact that he would make that same choice again, that's why Harry is slowly getting used to the self image of "I am someone who kills in cold blood when I think I should" - which is a dark place to be. Thing is, that if he hadn't had time to get used to that image, I doubt that he could have killed Susan.
Richard