A couple of points:
But the fact that there are no charges being pressed enrages me.
Are you a parent? There's absolutely NOTHING they could do to the guy that would be worse than losing a child. I wouldn't be surprised if he winds up comitting suicide intentionally, with the same gun. I can't imagine how much this guy's hurting right now.
I'd also betting his marriage is over. Yes, charges of child endangerment could be filed, but no punishment is going to change anything; no punishment that state can inflict will come close to what he's done to himself.
I am a parent, and my eyes tear up thinking about a child dying, mine or one I've never met - they are all tragedies. However, in this case the *stepfather* left a gun around that killed a child that wasn't his. He may or may not be suffering, and it should be investigated.
If all else fails, lower your standards.