I think you're exaggerating the circumstances in Even Hand. He didn't just barely not kill them. He had no intention of killing the child at all. If you remember anything about him, you should remember that kids are safe. As for Justine, she was a spy that cost him valuable resources and who's actions injured his people, to use him as a cat's paw. Is it really so outlandish for someone to consider such things against a potential enemy that abused his own sensitivities?
As I recall the story, in the end,
after the Fomor are driven off and Justine and the kid are safe, Marcone's inner monologue very clearly and explicitly considers killing both of them, because they've seen too much of his defenses.
Marcone is a bad person, who does bad things, for selfish reasons.
He is not fit for a Sword.
Admittedly, my copy is with someone else right now so I can't check it, but I do not recall Marcone excepting the child from his consideration of murder.
But despite all that, he said he debated it, and then decided against it. There was never a rational reason for him to do it, so he wouldn't have. That's the one thing you can count on for Marcone. He won't do something without a profit.
That he considered it and only decided against it for the reason of, "It means killing Harry now instead of killing Harry later," is not a good mark. He had to
talk himself out of murdering a woman and an innocent child.
His idea for killing Justine and the child was made for cold, rational reasons -- that they knew of his defenses and
might, some time in the future, tell Harry about them. He didn't kill them because doing so would mean fighting Harry immediately instead of later on -- i.e., he made the decision based on
when would be a good time to murder someone else.
As for the sword, it seems highly unlikely that he'd even be worthy. But it might be interesting to see him touch it while thinking about protecting his city, which he will likely defend as passionately and he does any of his people. Which is to say not passionately, but with cold determination.
Marcone doesn't want another predator taking away his prey. He doesn't have some altruistic, virtuous desire to protect the innocent. He's just defending his hunting territory.
