NPCs exist to fight or give quests to the player.
If the police suspect a PC of a crime, the PC will either steal the evidence or kill the police.
If the PCs like an NPC, they will demolish anything that threatens them.
The players will never like any NPC that GM intends the players to like.
If the PCs dislike an NPC, they will kill or betray the NPC, regardless of consequences.
I agree with all of these, but I would like to put another on there:
If an NPC is a dick, then the players will do anything to ruin that character's life (whether it be murder the NPC or just be really confrontational to him/her).
Even if the NPC isn't a dick, but the players perceive said NPC to be one, the NPC's life will be ruined.
Let's face it, did Thelonius have to die? No. We took that too far.
I think after awhile, we have done a pretty good job of fixing things and being less confrontational (i.e. dicks). I'll chalk it up to trying to survive in this harsh New World.
SLIGHT SPOILERS FOR THE NEW WORLD CAMPAIGN?
The moment I realized our group was too confrontational (i.e. dicks) was when I left for a session and came back to find that we wanted to kill someone just because they lied to us. I asked, "wait, did she take anything from us?"
"No."
"Did she try to kill us?"
"No."
"Then why are we trying to kill her again."
"She lied to us dude!"
Let's face it, our group has never been the most morally upstanding group of adventurers in the New World and yet, this person was hiding her true identity and now we have to kill her?