Killing necessary? Bhagavad Gita

I put this in the philosophy thread rather than the spiritual because I wanted to discuss the killing aspect rather than the religious.

Ok, first of all, I found it difficult to reconcile the apparent compassionless condoning of killing people in war with the compassionate condemning of killing animals for food in the Gita, though it could be argued that people choose to fight whereas animals don't choose to be eaten.  My general position is that killing should only ever be an absolute last resort where there is no other option and necessity reigns.  Yes, life can be a physical reality only where there is movement and change, birth and death, but can that ever justify wilful taking of another's life?  Suffering is suffering and death is feared by most, if not all species.  Sadly death will happen, but where do we all draw the lines, and can even a spiritual or religious philosophy be justification for ending a life?