WARNING: Plenty of movie spoilers in this article - specifically in regard to movie endings.
Was Patrick Bateman really a serial killer? Was Deckard a replicant? Did the top fall over?
The 'ambiguous' movie ending is the ultimate gamble, with payouts ranging from sparking lively discussion or adding a frustrating period to a two-hour investment of time.
All endings have a value. The most valuable - the ones worth analysis, discussion and debate - are statements about archetypal, existential or thematic truths. The most frustratingly vague endings tend to be 'circumstantial' - they leave into question details that don't really matter, leaving them bereft of substantial value.
The problem in leaving a conclusion open to interpretation is, the conversation can get stuck in a mud of specifics -- the 'circumstantial' -- that distracts from core thematic and existential values.
The key talking point of "Inception" was: "do you think the top fell over?" That's short hand for, "Is Cobb stuck in a dream or was he really reunited with his children in reality?"
But this specific situation is not nearly as interesting as his choices leading up to the end credits. Throughout the film, Cobb goes out of his way to covertly spend extended blocks of time in his own dreamscape - where he can visit Mal. Yet he also routinely establishes his dream/reality bearings by using his totem. It's as important to Cobb to dream as it is to know when he's not dreaming.
In the final scene, Cobb spins his top -- and immediately turns away to embrace his son and daughter. In the end, he doesn't care what the top does. Everything Cobb does has but one goal - to reunite him with his kids. Whether the top tips over or spins eternally essentially boils down to a single story value: is his final success diminished if he's reunited with his children only in his dreams?
Whether this particular top falls is not as important as the intrinsic value placed on dreams versus 'reality.' (See also, "Total Recall")
After surviving a plane crash and eluding a pack of vicious wolves for miles, in the end, Ottway discovers he has walked straight into the wolves' den. And just as he's about to fight the alpha male wolf, the screen cuts to black. Does Ottway survive his fight to the death with a massive wolf?
Ottway's ultimate fate doesn't matter as much as his ultimate choice: to stand his ground against an apex predator.
Up to that point, Ottway has only attempted escape - from life and from death. He wrote a suicide note and put a gun in his mouth early in the film. He literally runs for his life for the next 100 minutes. He doesn't want to live and he doesn't want to die.
But strung throughout "The Grey" is a poem written by Ottway's father - which directly addresses Ottway's very specific dilemma: "Into the last good fight I'll ever know ... Live and die on this day ..."
In the end, Ottway chooses to fight for himself. He prepares for a vicious fight-to-the-death. He decides life is worth fighting for - even if means facing a lethal enemy. Whether he dies fighting or survives the battle is a footnote to his much more important decision to bravely engage in the last good fight he'll ever know. (See also, "The Wrestler")
After butchering, shooting and raping numerous New Yorkers, Patrick Bateman confesses his many brutal, violent crimes to his lawyer as the police close in on him. The next day, Bateman discovers his oft-used crime scene lacks any evidence of murder and the lawyer laughs off Bateman's confession as a joke. In the end, Bateman goes unpunished for his perceived killing spree.
Is Bateman a violent serial killer or is he merely a sociopath losing grip on reality?
He matter-of-factly tells people he is into "murders and executions" and plainly self-identifies as a psychotic murderer. His secretary finds a notepad filled with detailed drawings of murder and rape. Yet, it's possible all the horrific on-screen violence - involving chainsaws, axes and exploding cars - is all simply in the head of an unreliable narrator/storyteller.
What is known are the elaborate lengths Bateman goes to in establishing/defining his appearances on all fronts - taking great efforts to have the very best skin, home theater systems and (especially) business cards.
Whether Bateman really is a serial killer or just a yuppie with disturbing thoughts doesn't matter. This man burns tons of calories to carefully define his persona - yet, in the end, he is no closer in resolving his primary conflict: identity. In his mind, Bateman can butcher indiscriminately and literally confess his perceived crimes - without any sort of consequence. His victims are believed alive - and no one questions his slipping "mask of sanity."
It's unclear whether Bateman actually is a killer - however, it's clear (at the very least) Bateman has all the thoughts of a sociopathic killer.
His inner thoughts and dreams reveal a more significant truth about Bateman's true identity than whether he's a serial killer with murderous thoughts or merely an investment banker with sociopathic fantasies. (See also "Blade Runner")
Generally speaking, 'ambiguous' endings to any narrative fall into one of three buckets: lazy, gimmicky or thematic. Lazy ones can be shocking and gimmicky ones can be fun - but their loose ends simply do not matter.
The specific words Bob Harris whispered to Charlotte in "Lost in Translation." Where Enid goes after boarding the bus in "Ghost World." MacReady and Childs sharing a tension-filled drink in "The Thing." Internal logic can connect every narrative dot in these vague endings - but untangling these mysteries in no way changes their final, thematic resolutions. Bob Harris leaves Japan alone. Enid has left her friends and family behind. Either MacReady or Childs (or neither) could be the shape-shifting creature.
In most cases, the granular details of any film's resolution ending does not matter. As odd as it sounds, the specific fate of any specific character in any given story every told is irrelevant compared to their choices - which usually reflect universal human conditions and timeless themes.
Characters of all dimensions and depth are merely conventions in storytelling. Their fate is incidental to their character decisions.
Ambiguous endings actually highlight this distinction between choice and resolution.
Once any viewer is no longer mired in parsing out the literal, the story's more important symbolic values can be pondered and discussed.