Which boiling it down basically means: A: The Black Dog told me to kill Bob and God told me that it would be OK if I killed Bob. Q: OK but if I kill Sue would that be good? A: Oh no you can't kill Sue, she's a very nice person. BUZZER sorry no Criminal Insanity because you know that it is wrong to murder people, even if you believe you have a special exemption from that rule.
But is it not a matter of TIMING? If I suffer from sporadic but debilitating back-pain, I can lift something heavy today but not tomorrow. Am I "guilty" for not lifting when I am in pain? If I am in an acute phase of my psychosis, I simply may not know right from wrong at THAT TIME, but at other times I do.
Also, your example may not prove anything for a few reasons. First, it would imply that a killer as to think that he has to kill EVERYONE, or want to kill everyone, in order to get off on an insanity charge. Second, I come back to the issue of timing or non-absolutes. My mental condition simply may tell me that some people are good, and some people are not. My mental condition makes it so I don't want to murder the good people, but I do want to murder the very bad. Further, my mental condition may make it so that I only think one person, instead of everyone is bad. I could re-frame the answer to your question such that I say "Oh no you can't kill Sue because she is a very nice person, but I KNOW I have to kill Bob. Murdering Sue is wrong because she is nice, but murdering Bob is totally justified because he will kill me.... he is the devil and I am right to kill him. The US killed Osama Bin Laden, didn't they?"
However, on the other side of the coin, if I know I have a mental condition that makes me dangerous, is it not my responsibility to manage it appropriately, commit myself, get proper help, have people around me who can isolate, medicate or commit me when I am about to go on a murder spree? Should I not take my meds, see a psychiatrist, etc... I would hope that this guy did not properly manage his condition and can be found guilty under civil rules.