I'm wondering as angels are strictly good beings. Can they fall in the Golarion setting? And if so what happens with them?
I don't have it in front of me but I seem to remember that the titular Herald from the Wrath of the Righteous Adventure Path's Herald of the Ivory Labyrinth was a fallen angel.
While not Golarion specific the SRD does explicitly mention fallen angels and since Golarion exists as an engine to exercise the Pathfinder Core Rules I would argue that even if there aren't any existing fallen angels in the current written setting their existence is not precluded.
Pathfinder Tales "The Redemption Engine" goes over the topic. This question was asked a long time ago, but if anyone needs it in the future, quick summary:
Yes, it can happen...
Angels can fall. As pointed by Niekell, it is strongly hinted that Erinyes are fallen angels. They are in fact also known by that name, but they could just spawns from Hell that were created in a mockery of angel.
Furthermore, in The Redemption Engine novel, the plot revolve around a devil that has ascended.
Namely, Arathuziel the Chained, a redeemed deimavigga. It offers a bit of lore about what a Redeemed looks like and what challenges one faces in the angelic society.
Such a thing is extremely uncommon. The reason being, outsiders do not have free will per se. Sure, they can make choices and act, but it is always strongly submitted to their nature. Unlike mortals, outsiders are pure embodiment of their plane of origin. Thus, they do not willingly and spontaneously decide to change. Only a major event in their "life" can trigger that transition.
For Arathuziel, a deimavigga dedicated to twist and shake the faith of mortals, the turning point is the shattering of his own beliefs in the absolute law of Hell and its perfect order (from his point of view).
Bit more of lore.
The transition mentionned in the novel, is neither instantaneous nor easy. The devil that ascended was originally some sort of insectoid monstruosity. After a major event,
(Angels going on a suicide mission in Hell, and one of them fighting the deimavigga relentlessy even when she had no hope to win, shaking his core beliefs in Hell)
he retreat to his tower and goes through a long process of "shedding" his infernal form.
His chitin falls off, his claws recede and become fingers. Finally, wings sprout out of his back, and he burst out of his former shell, like some kind of holy buttefly. His external appearance reflects his nature, and one could assume the same goes for a fallen (albeit it's up to the GM narration). He then has to flee Hell, lest he would be taken prisonner and probably killed or tortured (depending of the devils mood that day).
A devil or an angel undergoing the process of transitionning is probably lost/confused, vulnerable and doing so in (future) enemy's territory. Even such a transition should happen, the angel/devil may not survive the process and the flight that ensue.
An angel that has fallen may well not look like an angel anymore, and fully adopt the appearance of a devil/daemon/demon/whatever it has transformed into.
There are a few inferences that Erinyes are fallen angels, as follows
Some calamity has befallen this angelic warrior.
Known by many names—the Fallen, the Ash Wings, and the Furies—the devils called erinyes mock the form of the angelic hosts in their exaction of vengeance and bloody justice.
however it isn't clearly stated.