what i've noticed about this film that is often overlooked or unaccounted for is the message that it is not a 'devil' out there who made you do it, separate from people and therefore real possession from the devil. she is affected and possessed but not by the devil itself. it's humanities own evil. even today, most people assume this film is about demonic possession and that evil is a separate issue from people. this disassociation is to not take responsibility for humanity's own evil/ugliness/shame. pazuzu's appearance indicates only temptation which leads to a revealing of the darker aspects of those who take the bait, hence the fighting dogs. evil is given greater power by the complicity (actions) of people and the evil capability mirrored in them. this is evident by dennings (implied perpetrator) and regan (victim).
the clues are in two parts. the first, father damien told regan it was holy water, when it was tap water and she reacted violently and spoke english backwards. the second clue is when he asked who or what is inside regan, it said 'we are no one.' this implies just evil or lower quality disembodied spirits/souls (not the devil), meaning they could have been human at one time. she was sexually abused and there implies some transference of these evil entities torturing her.
if you pay attention to regan's actions and words after the so-called possession expressing it's evil - you hear the vulgarity/profanity, manipulation of lies mixed with truth, pleasure in taunting and suffering/discomfort/anguish of another, anger, ego, degradation, recklessness/uncaring, disrespect, immorality, sacrilegious etc. but this is shown blatantly and heightened in glaring fashion to drive the point home.
what these attitudes and words indicate are not inherently different than what people exhibit from their own darkness or evil, it is just magnified to show how ugly/hideous it truly is and put on screen. the proverbial 'who knows what kind of evil lurks in man' metaphor.
much of this behavior is often even publicly visible among the lower to average population (common) because it is considered acceptable, especially generally weak/hypocritical ethics, vulgarity, ego, general irreverence and lack of respect. besides this, from life experience, you will see glimpses and peaks of this evil in people from time to time from glee at others misfortune, jealousy/envy, perversion/immorality, callousness, hypocrisy, egoism, dishonesty etc which are often overlooked because it's commonplace. but technically, this indicates a seed of depravity and evil that resides that could potentially be worse if society allowed it.
in other words, the evil is part of people's nature whether a devil exists or not. to what degree or not is what differs from person to person due to nature and/or nurture.