Even if thought to be the defence of self defence to the charge of murder (as in, eating this person will save your life, not doing so will kill you), if is NOT ok to kill and eat someone. Also to eat someone who is already dead would be interfering with a corpse, which again is criminal (and rather putrid). Classic criminal law case that challenges one's philosophical understanding of what makes and act criminal and why culpability and vulnerability are significant considerations in deciding what behaviour us criminal and in the sentencing of offenders. For a really good discussion of the case see Mirko Bargaric, Kenneth J Arenson & Peter Gillies' book "Criminal Law in the Common Law Jurisdictions". Fantastic and thought provoking chapters on necessity.