She is dead, but not due to being murdered by the attackers (as Lenny, the main character, likes to delude himself.) She died of diabetes almost a year later.
He tells himself that he is out to revenge his wife against the attackers, when really he’s out killing due to anger over his condition.
In the comedy Arsenic and Old Lace, while the delightful old aunts are not the primary protagonists (I suppose that falls to Mortimer although in the play the aunts each have many more lines than he), as serial killers I suppose they could be called “bad guys” even though nephew Jonathon is much worse. Still the “twist” is that they are serial killers, but I think virtually no one holds it against them.
You’re right about Teddy being a bad guy. But right at the end of the movie, after Teddy tells him that his wife didn’t die, Leonard comments that he knows he is going to forget this, which will allow him to continue on his “crusade”.
My question is, is his wife really alive or is it one of Teddy’s lies, with Sammy being real? If the latter, the movie loses credibility - the idea that he could investigage a man with this condition and then develop it himself is about as fantastic as, well, the idea that Neil Young could have two children with cerebal palsy by two different mothers.
Well, now I see I misread your post. Thought you were saying that only Teddy was the bad guy.
The Baldur’s Gate video games arguably belong here. You, the protagonist, slowly discover you are the child of a particularly evil and brutal god, and you have to fight off the urge to follow in his footsteps by occasionally turning into a demonesque monster. Granted, the player really chooses his or her own destiny and can decide to be a good guy, but the story line is there.
It seems to me that it was fairly clear what happened. Lenny killed his wife with an insulin overdose because she was “testing” him in the same way that Sammy Jankis was tested by his wife - but of course, Sammy Jankis didn’t actually have the condition and was busted by Lenny as a fraud (and, I assume, never killed his wife, unless he was a really dedicated fraud). Lenny’s memories got corrupted in that he mixed and matched bits of Sammy with bits of Lenny.
Teddy, of course, knows the truth and TRIED to talk sense to Lenny before being killed - even though he knew it wasn’t going to work.
-Joe
I’d say Dune is an edge example, because although the Atreides family aren’t mustache-twirling villains like the Harkonnens and they aren’t portrayed as particularly evil, if you look at the things Paul does he’s a monster.
Another good example that I remembered from another thread is “My Best Friend’s Wedding”. Julia Roberts has to bust up the impending wedding of her soul mate in classic rom-com fashion. Except her classic rom-com deceptions and stunts would be the actions of a major league asshole if someone did them in real life, and at the end of the movie Julia Roberts realizes that she has indeed been a major league asshole.
Also arguably qualifying (depending on your interpretation of the film) The Hitcher.
Does the Twilight Zone episode where the guy turns into Hitler count?
That makes perfect sense. Was caught up in thinking that his memory of Sammy was either completely true or completely false.
I don’t think we really know much of anything about Sammy Jankis. Maybe he didn’t even exist. But it seems clear that lots of things Lenny attributed to Sammy actually happened to Lenny. Either that, or Lenny started attributing things to himself that actually happened to Sammy.
Anything shown in flashback is unreliable, and anything said by any character is unreliable. Teddy believed that there was a Sammy Jankis, and that Lenny proved he was a “fraud”, in that his memory loss wasn’t physical but psychological. But is Lenny’s memory loss physical or psychological?
In any case, at first Lenny seems to be a victim, struggling against his condition and trying to achieve justice for his wife’s murder. But we find out that Lenny really is a kind of monster, who is willing to leave himself clues that he knows he will later misinterpret. He’s willing to lie to his future self to get his future self to kill Teddy. And he doesn’t want the truth, any time he finds out the truth about what really happened to his wife, or what really happened to him, he destroys that evidence so that his future self won’t know.
And to top it all off, it’s suggested that Lenny’s case isn’t physical either, but psychological. He blanks out his own memories because he can’t live with the truth. Lenny is a monster because he runs around killing people for reasons he can barely understand. And what makes him more of a monster is that he turned himself into that monster on purpose.
Along those lines, the Knights of the Old Republic video game (the first one) fits as well. You play a Republic soldier who turns out to have a talent for the Force, and are trained as a Jedi to help you and your team defeat an evil Sith Lord who wants to destroy the Republic. This Sith had turned on his master, Darth Revan, taking advantage of the Jedi attacking Revan at the same time, and Revan was killed in the crossfire.
Except, you turn out to be Revan. He/she was only badly injured and comatose, and the Jedi created a new personality and memories and implanted the body with them, creating you and hoping you’d lead them, following these “visions from the Force” to Revan’s old source of power that the new Sith Lord had taken over. After you find out you got lied to by the Jedi, you can opt to screw them over and try to reclaim your old mantle as the Lord of the Sith.
IIRC, Morgan Freeman’s character in Chain Reaction (1996) fits the OP to a tee. Our heroes, Keanu Reeves (Og help us) and Rachel Weisz are on the run, framed for murder. Freeman, Reeves’ mentor, helps as much as he can. Only, it turns out … (see the plot summary).
Oh, well, crooked mentors (as opposed to protagonists), that’s a whole other can of worms. (L.A. Confidential, Unbreakable…)
No, no, not KFC, **KGB **! 
Or maybe the KGB KFC, I can’t remember…
Okay, almost embarrassed that I remember this one, but I flipped by it the other night: “Shattered”
No one’s mentioned Heroes yet?
Come to think about it, there’s also plenty of times in Lost where the good guy ends being the bad guy. only to become good and/or bad as the seasons go by.
Of course, by “Agatha Christie” you mean “Sheridan Le Fanu.”
In the Sten series, after seven books, the hero finally realizes that the Emperor he’s been working for isn’t really much if at all better than the people he’s been fighting. And that all he’s been really been doing is protecting and promoting a tyranny with a better sense of propaganda than most.
My memory’s very vague, including about the title. But there was a story about a government agent hunting down an android in his likeness, that was supposed to infiltrate his planet, then detonate a planet cracking bomb by speaking certain preprogrammed words. Finally, he corners and shoots the android, who lies on the ground, spurting blood, and not oil or sparks. And he says, “But if he’s the real one, then I’m” - and the explosion can be seen a million miles away.
In the webcomic Goblins, there’s a scene where the matriarch goblin points out to one of the adventurers that whatever other goblins have done, this particular village hasn’t attacked anyone. The adventurer looks around with this horrified expression at all the dead goblins, and the maniacal looking adventurers chasing the survivors, and says, “but . . . but . . . we’re the good guys . . . right ?!”
H.P. Lovecraft, The Outsider. Where the protagonist at the end discovers that the hideous undead abomination that everyone is running from - is a mirror.
T and I, where the protagonist discovers that he is the artificial alternate personality of T, a goodlife ( collaborator with Berserkers, anti-life war machines ), rapist, murderer and all around scum.