Hey TIS, when you really think about it Negan can't kill all of them because he needs people to go out and gather goods and supplies, as well as fend off others that may be out there, in order to survive.

Robert Kirkman made an excellent point when he was on The Talking Dead when he said ( and I paraphrase now ) " Had viewers been introduced to Negan from the pilot episode and grown with him and his group throughout the seasons, the viewers would be viewing Negan and his group as the good guys and the new season 6 character Rick Grimes and his group as the bad guys who went around slashing throats and stabbing people in their sleep."

Great point and I agree.

What it comes down to is that everyone in this post apocalyptic world is doing what they need to do in order to survive. It's the living that now pose the major threats, not the walking dead.