Let’s start by putting this into some context. The game I am currently running is a Savage Worlds - Necessary Evil campaign. This is their version of a superhero game wherein the all the good guys were killed off by invading aliens and it is now up to the villains of the world (the player characters) to retake Earth. The setting is near future; a modern world with comic book sci-fi in it (laser guns, etc). In this setting, a nuclear weapon is the pinnacle of weaponry. With a nuclear bomb the characters can kill anyone they want, and a lot of them.
So, back to the original question; why did I give them such a weapon? The initial plan was to present the players with a moral quandary. There is this supervillain who has recently found out the leader of the alien invasion takes psychic damage whenever people within a certain city die (the base city of the campaign and one built by a different alien under mysterious circumstances). In fact, the more people that die there, the more damage the alien leader takes. So this supervillain has decided to kill everyone in the city and hired the player characters to recover a nuclear weapon from a launch silo that was not completely destroyed during the invasion. The PCs know all the background information, thus they were presented with a moral dilemma; use a nuclear bomb to kill the alien leader and 4 million humans and possibly stop the invasion - or - not use the nuclear bomb and protect 4 million human lives.
The characters ran the mission and made off with two nuclear weapons that were still usable from the silo. And then they told the supervillain that the silo was empty of nuclear weapons that would work. They chose to not use the weapon and protect the lives within the city. But, they kept the nuclear weapon, mostly because they were fascinated by the concept of having a nuclear bomb.
Now I could have decided that there were no usable nuclear weapons within the silo. After all, the moral dilemma had already been presented to the players and the players had already made their decision. But then I was intrigued by the follow-up question; what would the players do with the most powerful weapon on the planet at their disposal? So I let the game go as written.
As a player, I would never expect a GM to hand over such a powerful weapon to the player characters. Doubtless, neither did my players. They probably assumed the weapons would be unusable; basically they were only the draw to run an adventure but that the characters had no real chance at getting a nuclear bomb. And then the truly unexpected happened. They got the most powerful weapon in the world…in their hands…to do with as they desire.
And that is sort of the point of this post. Throw something completely unexpected at the players and their characters. Do this on a meta-game level as well as at the in-game level. There are some meta-assumptions all gamers make. One of them is that the players will never have the ultimate weapon (in this case a nuclear bomb) that functions and that they can use without restriction. Think back to how many adventures and stories revolve around recovering a powerful artifact only to have the artifact be destroyed or otherwise not be recovered (falling into a deep crevasse seems to be the favorite method for doing this).
However, sometimes it is desirable to do exactly that – give the players the very thing no GM in his right mind would give the players. Sometimes we as GMs fear the repercussions of allowing this to happen. “It will destroy the campaign I have spent hours and hours designing!” So what? Let it. Let the players do as they will; the game really is about them after all.
So in conclusion, don’t be afraid to let things go where they will. Don’t second guess your players and deny them the world.
And what did my players do with their nuclear bombs? They buried them in 20’ of concrete below their secret base because they don’t want them to fall into the wrong hands. They may eventually end up using them, but for now they are scared of them – as any player character should be.