More likely, the charging authority does believe that there is sufficient evidence to prove beyond a reasonable doubt that the man's actions were
not lawful.
At the arraignment hearing, the charges were reduced to two counts of assault, harassment and criminal possession of a weapon.
Right. I agree. Bringing charges of any kind means the DA considers at least some of the husband's actions unlawful, i.e., not justified.
I'm addressing the original question as to why the alleged killing of the alleged would-be rapist is prosecuted at all, i.e., why the DA does not simply consider it justifiable. In broad terms, justification for a killing requires proof of fear of some impending harm. We are likely seeing the evolution of some mitigating factors (see my earlier post).
But if the alleged would-be rapist was just sauntering casually away, it's hard to see a case for justification for killing him. If, however, the alleged killer had just been apprised that the alleged would-be rapist has just committed a violent aggression and then takes the first swing, a more colorable picture of justification begins to emerge.
True justification, however, suggests no prosecution at all, not reducing charges to assault. Of course, there can be all kinds of other charges, like the weapons possession charge, etc. Does the husband have a criminal record? Does the wife really not know the attacker? Who knows?
The recently-elected Bronx DA will want to get this right. She has a lot riding on her response to this high-profile case. Homicides can't be taken lightly. Neither can attempted rape. Various community interests are involved. It's a political minefield. She's trying to gather facts as quickly and as reliably as possible.
It's early days. We would need solid facts, and plenty of them, to draw meaningful conclusions. For now, we can only offer generalities. But these can at least be firmly grounded in law and criminal procedure.