In North Dakota, one is legally permitted to use lethal force to intervene on behalf of a third party to prevent/mitigate the serious bodily injury or death of that third party.
Rape is included in the definition of "serious bodily injury".
Of course my wife is faithful, ergo, the intervention would be justifiable under the letter of the law.
No insanity defense is necessary, instead it would be the lawful use of force on behalf of one who is being seriously injured.
Laws may vary in your jurisdiction. Learn them.