They may try an insanity plea; however, if he knew it was wrong to kill at the time of the incident, he's done.
That alone isn't enough. It can actually bolster his position. "I knew it was wrong, but I couldn't stop"......
I've seen the "did you know it was wrong" bit in too many insanity defense cases and it didn't matter.
Sad, but true.
Jonathan
http://legal-dictionary.thefreedictionary.com/irresistible+impulseCongress and most states abolished the irresistible impulse defense after John Hinckley was acquitted on grounds of insanity for the attempted assassination of President ronald reagan in 1981. Only a handful of states currently allow irresistible impulse as a defense to criminal charges. These states permit it as a supplement to the cognitive insanity defense, which is the only insanity defense recognized in most jurisdictions. On the federal level, Congress abolished the irresistible impulse defense in the Insanity Defense Reform Act of 1984 (18 U.S.C.A. §§ 1 note, 17).