You know you want to fuck the players over with NPC treachery and cowardice. Admit it. It's half the fun of being a DM. Nothing warms my heart like when Dave the Torch-bearer loses his shit and runs off in the middle of the big fight carrying the only light source. Or when the porters steal all the PC's shit and wee in their rations.
Best part is, if it's done right, it's the players' fault, with many players using pack bearers as meat shields & trap-springers, etc.
So here's some rules to help you justify this sort of assbaggery. These rules are still under play test conditions, and comments are welcome.
Loyalty & Morale Checks for NPCs:
NPCs are not robots that fight to the death, or put up with any amount of abuse from PCs. Periodically, and when the situation calls for it, a loyalty check must be made to ensure the loyalty of the NPC. Additionally, in dangerous situations, the NPC must make a morale check, which is a modified loyalty check. PCs and cohorts are always assumed to have passed these checks, unless magically compelled.
Base Check: The basic loyalty check involves the NPC making a will save against the Charisma of the party leader (or the PC who hired him, if they're not the same person), or the person who wronged the NPC. The base DC of the check is the Charisma score (not modifier) of the PC in question plus the morale score of the NPC in question, with modifiers listed below included. If the PC has leadership, he uses his leadership versus followers score instead, if it is higher than his Charisma.
If the NPC makes the save, he becomes disloyal. For good NPCs, this may mean anything from sulking to quitting. For Neutral or evil NPCs, it means sulking, quitting, or treachery. Sulking means the NPC is considered shaken until morale has been reestablished, and will attempt to sap the honor of the person who offended him. Quitting means exactly that. As soon as the party is in a safe place, the NPC resigns and leaves. Treachery means the NPC will try to take revenge on those he feels has wronged him.
The base morale check is the same, except the DC of the encounter (or the worst encounter so far in a given session, if there is no immediate combat) is subtracted from the target PC's charisma. If the NPC makes his save, he stands frozen in panic. If he is struck, he flees. If he is not struck, he must make an immediate loyalty check after the battle or situation.
When to make a check: A loyalty or morale check is made when the negative modifiers totaled up exceed the NPC's morale score. Also, an NPC placed in a position of temptation may have to check, at the discretion of the DM
Modifiers All of these modifiers stack.
Temporary modifiers are based on the current (and possibly past) situation.
Pay Modifier to DC
Less than one half book -2
Less than book -1
Book 0
More than book +1
Double book +2
Triple book +3
Pay is in arrears -3, pay modifiers are not used.
Each offer to raise pay forces a re-reroll on a failed save, provided the new pay scale is higher than the last one and at least book. A new check can be forced for each offered raise, though if the raise goes up more than one bracket on the first attempt, it gives a +1 for every bracket jumped. Pay in arrears must be made up before any chance of recovering loyalty, unless a better offer is made and a DC20 diplomacy check is made to convince the NPC to defer his pay.
Treatment: An NPC who is well treated (DM's discretion) gets a +1 to +5 modifier to the DC. If an NPC is raised, he and all other NPCs take a permanent +1 to the DC, in addition to the bonus gained from the increase in leadership score, if appropriate. Poor treatment has the exact opposite range of modifiers, and if an NPC is left to his death, the other NPCs take a permanent -1 to the DC. If a PC kills an NPC, the other NPCs immediately gain a -5 modifier to the DC and make an immediate check. EXCEPTION: If an outright mutiny is in progress and a PC harms a disloyal NPC, an immediate loyalty retest is made, with a success meaning the mutiny spreads or gets more daring, and a failure meaning all surviving mutineers sulk. They are disloyal at this point, but at least they aren't in mutiny.
Morale check modifiers are as per loyalty checks, but have additional modifiers to the DC. A morale check is made when the value of all modifiers (loyalty and morale) exceed the NPC's morale.
Encounter CR = +5 or more the PC APL: -5 (does not stack with +3 APL)
Encounter CR = +3 or more the PC APL: -2 (does not stack with +5 APL)
Encounter CR = below PC APL: +2
Per PC fallen or fled: +2
PC who hired PCs fallen or fled: +4
Per NPC fallen or fled: +1
NPC is wounded: -1 (Remains for duration of combat, even if healed. Stacks)
PCs cure NPCs: +2 (Remains for duration of combat. Stacks.)
NPC Morale Score: An NPC's morale score is based on his personality and his alignment. Law and Good each grant a +1. Evil and Chaos each grant a -1. NPC traits and goals from Game Mastery Guide, pages 94-96, may add or subtract to the score as determined by the DM, ranging from -2 to 2.
NOTES: The same rules can be applied to monsters, with the master of the monster taking the place of the PC, or if there is no master, the check is equal to 10 plus the CR of the encounter.