The movie version of A Clockwork Orange was based on the American version of the book, which left out the entire last chapter. In that chapter, at 18 years old Alex pretty suddenly grows out of his violent and criminal ways and wants to start a family. Some say this ending is more optimistic but I actually think it's darker, because it shows that any normal person you meet might've at some point been a wanton brute reveling in the chaos and pain they so arbitrarily inflicted. And that they can just move on and start living like a normal person.

The Lawnmower Man

In the book, an unassuming everyman stumbles upon the fact that a local landscaping company is actually a front for a demon who has an arrangement that involves making human sacrifices of those that discover his supernatural nature.

In the movie, a Cyber Virtual Reality 3D Battles ON 3D CYBERSPACE Stunning Effects 3D Internet Pierce Brosnan Warfare Nineties Futuristic VR Headset Technology BATTLE In 3D Mind Expanding Guns, and one of the characters is a man who has a lawnmower.

Edit: Shit, okay, I just read this on Wikipedia and nearly wet myself:

A feature film, The Lawnmower Man, starring Jeff Fahey and Pierce Brosnan, was released in 1992 by New Line Cinema. This film used an original screenplay entitled "CyberGod", borrowing only the title of the short story. The film concerns a scientist, Dr. Lawrence Angelo (Brosnan), who subjects mentally challenged Jobe Smith (Fahey) to virtual reality experiments which give him superhuman abilities. The film was originally titled Stephen King's The Lawnmower Man. King won a lawsuit to have his name removed from the film, stating in court documents that the film "bore no meaningful resemblance" to his story. King then won further damages in 1993 after his name was included in the home video release.

I, Robot.

Asimov was explicitly trying to get away from the trope of "robots take over humanity". To be clear, the first short story that became I, Robot was published in 1940. "Robots take over humanity" was already an SF trope by then. Hollywood comes along more than half a century later and dives head first right back into that trope.

Lt Cmdr Data is more what Asimov had it mind. In fact, Data's character has direct references to Asimov, like his positronic brain.

I Am Legend

The ending was completely and utterly different than the book, which destroyed the gut punch at the end of the book that was kind of the whole theme of the book.

I don't even remember the book as a whole. But I remember the ending. Then they Hollywooded it and it was awful.

I know we're not into Harry Potter now, but the past is the past and I can't forget how annoyed I was when the movie based on the third book, Prisoner of Azkaban, came out. I was a very disappointed teenager.

It was a whirlwind story to me at the time. I remember exactly where I was when I read it, as the moment that revealed the friendship between Harry's father James, Professor Lupin, Peter Pettigrew, and the alleged-murderer, Sirius Black, became seared into my brain. It was such a pivotal part of the overall story to me, that that twist alone made it my favorite in the series. So when the movie came out, I expected the use and development of The Marauder's Map to be a key highlight. It was a huge deal in the books, after all.

Yet in the movie, the map is just a neat thing Harry gets to use. Nobody mentions that Harry's own father helped create it. The movie never even tells who the Marauders are, even though the reveal of their backstory was the key emotional crux of the Shrieking Shack scene. To omit their story entirely felt like a gut-punch.

I didn't understand at the time why the director (Alfonso Cuaron) decided to straight-up change everything that made that story so compelling to me and my friends. To this day, I still don't understand.

The Hobbit. Like, all of it

Stephen King - Dreamcatcher

In the book the character Duddits had the shining, yes that motherfucking shining.

In the movie they made him an undercover alien. Man what a let down.

The Hobbit

From the shitty shoehorned romance to wholesale elimination of plot points in the original story. Yeah, there was definitely some drama in the whole production of the film, but nonetheless it was crap.

The Dark Tower. Everything. An 8 book series smashed into 1 terrible movie. Who ever green lit that should be fired.

Literally everything about World War Z. Absolute travesty. The book is a unique and genuinely thought provoking new take on the zombie genre. The movie is an insult to every bit of world building Max Brooks created.

The book Annihilation centered on a "tower" that was a mysterious, fleshy, downward spiraling tunnel with creepy writing on the walls. The imagery was so unsettling.

For some reason it is entirely absent from the movie. Like... that was half of the point of the book - a "tower" that climbed down into the earth instead of towards the sky. Why would you cut that?

All the adaptations of I Am Legend are bad, but 2007 movie was insulting. It gave the illusion of following the book, but then did a u-tutn and completely changed the meaning of the story and the title itself.

In the movie the protagonist becomes a legend because he sacrifices himself to cure vampirism.

In the book he is the last man in a world of vampires, he kills vampires, and understands that he is like a legendary monster that kills people in their sleep. He is then executed.

Ready Player One. So much about the movie adaptation of this book infuriates me, but the fact they replaced Wargames with the Shining is a crime against humanity!!!

vaguely gestures at World War Z

I Am Legend. Just the whole thing.

Everything in WWZ

Maybe not the worst, but this one's personal: Edge of Tomorrow's take on the fantastic All You Need Is Kill (spoilers ahead).

• Making the movie PG-13. In chapter 2 of the manga, there is a brutal death scene showing how Keiji can't escape the Mimics wherever he goes. The series was quite bloody, and used that to its advantage.
• Casting Emily Blunt as "Rita Vrataski". One of her defining character traits was that she was unassuming, and that you wouldn't expect that level of combat skill from her appearance.
• While Keiji was in love with "Rita" in the original, it was unrequited–the change felt actively detrimental to "Rita's" character.

SIDENOTE: I feel like changing this was sort of unimportant, but you'll notice I'm using quotes for "Rita". That's because, in the original, her real name is unknown. She took someone else's identity.

Seek the truth, always.