Just checked my copy -- he doesn't mention killing the child, just Justine. And his reason is that she'll tell Dresden the details of his anti-Dresden defenses. (In Peace Talks, we learn that she did exactly that.)
In fact, he explicitly warns Justine about taking the child to the White Court, and suggests Father Forthill as an alternative.
The passage in question:
I debated putting a bullet in her head but decided against it.
But, yes, Marcone is not a good man. His role in the story is to show that the "rational, honorable monster" that Dresden often finds himself dealing with (like Lara and Mab) isn't an exclusively supernatural phenomenon. Marcone is fully human, fully rational, and fully a monster. And still probably better than what would replace him if he's killed -- just like Lara or Mab.
I don't think he'll be taking up a Sword, any more than Lara or Lea or Mab, without a serious redemption arc.