I like this. Adventurers would be a 'faction' and killing them (and unsuccessful quest completion, etc.) progressively drops your standing with them:
-their hiring cost increases, and cost to be hired by any enemies of yours decreases
-they refuse to be hired
-they avoid you
-they attack you
Treating them well (hiring, equipping, successful quests, etc.) increases your standing with commensurate benefits.
Actions must have consequences.
Good idea, and the bolded part is very important. If you merely increase hiring cost, the situation becomes worse - I couldn't afford to hire the last guy, so I killed him, now the next guy comes along and wants more - what do you think I'll do? I'll just kill them all, I can't afford them anyway, I may as well deny them to my enemies. Having them attack the player is similarly problematic; I was going to kill them for xp anyway, if they start attacking me it just makes killing them off much easier. Both of these punishments do nothing to players that have written off hiring NPCs at all, in fact they encourage more killing.
But if killing neutrals decreases the costs for your enemies to hire them, perhaps even to the point that they're flocking to your enemies for free if you slaughter enough, this changes the entire dynamic. Killing them is no longer a valid way to keep them away from your enemies, it accomplishes just the opposite.
Not that players should never kill the neutral NPCs, I don't want to see that possible playstyle taken away entirely - it should just have consequences, real consequences that do not encourage even more NPC-killing.