Let us talk about extra planar evils. In the Pathfinder universe, maybe a wider subject than you realize.
While I might enjoy a lengthy diatribe into the nature of evil in relation to the universe and planes of Pathfinder I think we should start off with a basic comparison, and that is the difference between devils and demons.
I suspect many may already know the basic differences if they have played tabletop rpgs for a while. Let us start with the premise devils and demons are different creatures. In our world we have The Devil, capital D. This is the Christian adversary, master of evil often associated with lucifer the fallen angel of the lord. Then there is a devil, lower case d which is considered a minion of hell.
As for Demons in our world there is not a capital D demon. Definitionally demons are evil spirits that posses and torment humans. Lower case d devils are used interchangeably with demons.
In pathfinder that is not the case. The two creatures while evil and out to do very terrible things to player characters have different traits, abilities and personalities. These divergences in evil philosophy mold the game mechanics and themes of each.
We shall first look at demons. What makes a demon terrifying is that they start with a mortal soul. When a mortal soul is judged by Pharasma (goddess of birth, death and fate) and sent to the abyss the weight and nature of their sin transforms the soul into a demon. The deep-down dark of the mortal soul becomes the seed that when cultured and tormented in the abyss of chaos reshapes the self into mockery of life and celebration of the core sin. Demons are spirits of desire and sin manifested on the outer plane seeking to indulge and destroy for the pleasure of the act. You do not reason with a demon, you do not make deals with a demon. The only limiter of a demon is the drive to continue their revelry in sin and the self-preservation instinct. Force and fear are the only restraints on a demon.
How about devils? If demons are the darkness the underlies the ego, then Devils are the darkness of the super ego. Devils adhere to laws and twisted power-hungry societies. Rules and laws are tools for the goal of corrupting mortal souls to make a valuable point. What is their point? I hope your GM understands evil enough to find one. The better the reason the more fun your games can be. Devils, because of these rules can be bargained with, entreated with, and some believe even neutrally admired.
That is the terrifying aspect of a devil, everything will seem logical, reasonable, like it was always your intention from the very beginning. It will only be your mistake, the flaw of being mortal that damns your soul to hell. If you the mortal player were of better character, knew not temptation you never would have agreed to take the devil’s hand out. You could have made the hard choice but you didn’t, and now you have crossed the line and can never go back. Devils will take their due because you signed the contract.
On Golarion demons seek absolute destruction of every mortal soul and potential thing of goodness. Devils seek to control mortals for their own ends. Typically this is planar conquest and control. Mortals are caught between two evils when devils and demons cross paths. Demons care nothing for rules and bargains and will sate their desire on any target. A tool that does not suit its function is of no use to a devil and should be annihilated. The two creatures do not get along. Mortals are often left with choosing the devil they know instead of a demon they can not fight. Remember this when you start to judge those from Cheliax and the choices they have been forced to make.
Narratively speaking these creatures provide lenses at which to examine characters and player choice. Demons work well as allegories against sins and overindulgence. Devils help show that control taken too far can justify any sin within the confines of the rules. Golarion is not a place of endless comedy sunshine and rainbows. The writers have taken painstaking efforts to represent the spectrum of the human condition amid magic, dragons and very real gods, angels and devils. Taffy, Stheno, Bread and Demos inhabit Golarion. We together discover their adventures with every episode and learn more about them with every choice. As the GM, my goal is to continue to present these characters with challenges and opportunities to test their metal. I am to evoke the core of those characters in my players so that they are as real to them as my voice is to you listening. Will they become devils or demons? Maybe they will become angels? I do know that they are mortal because at any moment one roll of the dice their fate might end with a rapid meeting with Pharasma Goddess of Death.