Remember that flashback scene in The Lord of the Rings when Elrond and Isildur are right at the center of Mount Doom, and all Elrond needs to do to save Middle-earth from centuries of war is elf-kick that dipshit and his ring-coveting ass into the boiling lava of oblivion? Had he just done what everyone in the audience was screaming at him to do, the movies would have just been 10 hours of hobbits getting drunk.
Well, this kind of thing happens all the time -- one poor decision leads to an entire film series worth of conflict. For instance ...
5Batman Exists Because Ra's al Ghul Doesn't Screen His Applicants
Christopher Nolan's Batman series begins in a Bhutanese prison, where Liam Neeson's Ra's Al Ghul (who apparently has a key) recruits Bruce Wayne to be trained as a member of the League of Shadows. Ra's takes Bruce under his wing, engaging in an array of standard training exercises, including hallucinogenic ninja hide-and-seek and ice dueling peppered with damning accusations about one's father. He becomes a mentor, teaching Bruce that in order to take on his enemies, he must transcend living as an ordinary man and become an incorruptible symbol (evidently Bruce hears this as "drive around in a car that explodes into a motorcycle").
Sadly, like anything good, there's always a catch -- and after Bruce's training is complete and he knows all of the League of Shadows' secrets, he finds himself being asked to execute a man in a show of loyalty.
"Come on, you're embarrassing me in front of my minions. I told them you were cool."
He also is told that Ra's is determined to use Bruce to facilitate the total destruction of Gotham City, something he waited until just now to mention. This doesn't sit well with Bruce, and because he's already balls-deep in the organization, he ends up having to explosion-stab his way out of the League of Shadows, destroying their headquarters in the process.
The best way to show your commitment to not killing your enemies is by blowing them up.
But hey -- free training, right? And now Bruce Wayne gets to be Batman, all thanks to Ra's' total inability to make sure Bruce is cool with murdering people before training him how to bleed out of the shadows and beat the screaming Jesus out of everything on the planet.
Seriously, Ra's -- it's called a job interview, and even the freaking Olive Garden does it. They're not going to start training someone before asking if they're cool with serving soup, salad and bread sticks. Ra's is planning to kill not only an entire city, but the city that Bruce Wayne was born and raised in and where everyone he cares about still lives. Why would he just assume that Bruce would be totally fine with it? This couldn't have been mentioned while they were making Bruce stutter-step on logs while beating him with sticks?
Apparently the League's motto was "Create unstoppable ninjas first, ask questions later."