Pulling the trigger.

[Timelord]

From that 600,000 you would have to remove the elderly infirm, physically incapable, small children or incapacitated for whatever reason - variable to the SHTF scenario. It will still leave a lot though but every geographical area will differ.
If you are wondering over the question of whether you could take another human life and are wrestling with this mentally, then you will not find out until the moment happens. If you are already genuinely confident of being able to carry it out then you probably won't be wrestling with the dilemma. Desensitization is the normal training method practised to overcome this "normal" human dilemma. Military practice this desensitization and the majority of console & PC games also are in this vein, which is indeed worrying! If you are worried about whether you could do it or not, then desensitization training would be required to change this to be confident of being able to carry the action out. (that is not false self deluding confidence but of actually being --mentally at ease-- with the idea of killing another human being - for individually morally acceptable SHTF reasons of course!!) I am not condoning any aspect of this thread, I am just discussing emotion & practicalities of the dilemma in a hypothetical third person sense..
For those that don't have any concept of the dilemma and don't give it a second thought after it being put to them, than they probably fall somewhere in the 600,000 category. Best avoid! lol. TL.