In my games it has been simple. The PCs often do get burned however for being to sympathetic, and as member of the Evil GM's guild I use it lest I be dismissed
Usualy the evil doers will come on strong, but they have little staying power and run away quickly. Whereas a village militia where are they going to go? They will stay till the death since they will be defending their homes and families.
As for a small hunting party, shots may be exchanged, but a small party of hunters or civilians who are not out for a gunfight will not be overly enthusiastic about getting into a protracted firefight and get their number killer or wounded, or, waste valuable and hard to come by ammo. So civilians <legitimate> would be open to parlay, they may even initiate such, or they will run after the first couple rounds.
Bandits as I said, would usualy be less organized and much better armed. Their attempts to parlay would be noticed and more for selfish purposes, as well as a trying to pull something over on the PCs.
As for "other" irregular forces. In my European games I employ ALOT of cavalry and partisan type forces. The cavalry usualy supply their own horses <yes they are available with a wave of my magic GM wand, of course this iss farm country> But, they are issued semi modern weapons and they act as auxilaries to the main first string Pact Forces. They patrol the roads, patrol the villages etc, they mopp up stragglers <like those from the 5th> downed aircrewmen and keep LRRP and similiar forces on their toes. These guys have killed three PCs and 2 primary NPCs <all unplaned> in one of my campaigns. These guys are treated like regular soldiers when encountered. Sometimes they are shot when captured but usualy they are left tied up which is often the fate of most enemy who are captured.
Partisans, they are shot outright, sometimes tortured. Since I handle partisans as guerillas and well as history has shown most guerillas are nasty people to begin with, often they will have done some nasty things to other allies the PCs may have found afterwards, a village that was freindly or even a PC or NPC so often the PCs have a reason to deal so harshly with these types.
We had one criminal join our group in one campaign. He caused many PCs to be killed and wounded and forced the PCs to fight their way out from a minning complex. They caught him with a radio and caught one of the enemy attackers who had info. The PCs put 2 and 2 together and found out their spy. 1 of the PCs wanted to shoot him, instead they hung him, heaving him on his tiptoes till the rope was taunt and left him. One PCs then shot him in one of his legs so he had to stand on his tip toe perfectly balanced lest he slowly strangle, and of course trying to keep his balance he would fall constantly with the noose choking him, the only way to get leverage was to lean into the noose have it choke some more and get his good leg under him and try to use his shattered leg as well which would cause him to fall again and repeat the process.
As Jack Sparrow said the inner circle of hell is reserved for mutineers and traitors.