I'm neither a lawyer or play one on TV, but I'm gonna call BS on that.
Defending oneself from an attacker is NOT murder.
630:1-b Second Degree Murder.
I. A person is guilty of murder in the second degree if:
(a) He knowingly causes the death of another; ...
II. Murder in the second degree shall be punishable by imprisonment for life or for such term as the court may order.
The elements are: "causing the death of another" and "knowingly." You don't "unknowingly" kill someone who's attacking you.
The State of New Hampshire, like many other states, can thus charge you with Second Degree Murder if you kill someone in self-defense, even if it's a crystal clear-cut open-and-shut justified self-defense, and force you to mount an affirmative defense under RSA 627:4.
Most DAs won't waste the effort filing charges in clear-cut cases, but perhaps the prosecutor is running for re-election that year, or is a media hound, or the case isn't quite so clear-cut from a certain warped perspective, or you're sleeping with his ex-wife, or something along those lines. Lacrosse, anyone?
This principle is spelled out in 627:1 - "627:1 General Rule. Conduct which is justifiable under this chapter constitutes a defense to any offense. The fact that such conduct is justifiable shall constitute a complete defense to any civil action based on such conduct. "
It's a defense to any offense. Not an elimination of the offense itself.