There are a lot of new ways to live in the spectacular futures of science fiction. Human beings can be placed in circumstances that we couldn't imagine today, for better and for worse. For example, the wars of the future could be fought entirely by people built in a lab and designed to kill.
A lot of sci-fi storytelling is about identity. Can a machine overcome its programming and attain something real? Does replacing one's body with robot parts change a person's humanity? This trope asks whether a person crafted to be the perfect killing tool can ever be anything else.
A human weapon is a person who has been somehow modified to serve a military purpose. This could be as simple as grafting weaponry onto an existing person or as complex as a full DNA rewrite to turn the soldier into a monster. This trope compares interestingly to the Super Soldier trope. The main difference is in the way the human weapon is treated. A super soldier can be the face of the army they represent, the beloved hero of the war. Human weapons are almost always treated more like objects than people. The military forces that equip human weapons with their tools and powers tend to see the weapon more than the human. If the human weapon is the hero of the tale, they'll likely be unhappy with their bloody lot in life. If they're just henchmen, they may be viewed sympathetically, but they'll probably still be butchered by the thousands. A single human capable of overwhelming destruction raises a lot of interesting questions, many of which have been asked by classic tales.
Arguably, this trope has its basis in real life. Many real human civilizations marked out a portion of their youngest as elite warriors. Most notably, the Spartans of Ancient Greece would select soldiers as young as seven and build their entire life around becoming the perfect warrior. This nightmarish combat grooming would almost always result in people who were psychologically and emotionally broken, but who could effectively kill others when necessary. A lot of ancient cultures treated their soldiers with a ton of respect, but that rarely balanced out the suffering they experienced throughout their life. Sci-fi takes the concept of a person being molded from youth to become the perfect warrior and adds the fantastical technology of the future. It's just as morally unforgivable, but far more effective.
One of the best-known and most consistently popular examples is unquestionably Marvel's Wolverine. In the world of the X-Men, plenty of people are born with incredible mutant powers that could make them more effective in combat than entire armies, but Wolverine went above and beyond. Young James Howlett was born with his trademark healing factor, which allows him to survive anything, live for centuries, and shrug off injuries almost immediately. He was also born with a set of bone claws that were slightly less effective than a regular knife. After decades at war, Howlett is brought under the knife and violently turned into the Wolverine. The resulting man-shaped monster is one of the deadliest beings in the Marvel universe. Contrasting Wolverine with a character like Captain America is the perfect way to understand the difference between the super soldier and the human weapon. Luckily, Wolverine found a way to use his unwanted gifts for good.
As one might expect, video games have made great use of this trope in both heroes and villains. Perhaps the most notable example is the Metal Gear franchise, which features human weapons as a central theme. One of the most important meta-textual aspects of the franchise is the way every soldier is treated by the world of conspiracies, politicians, and underworld voices that really run the show. Solid Snake and Raiden weren't just trained from birth, they gradually submit to more scientific tinkering with their bodies to make them more effective. As with most things in the franchise, Metal Gear Rising: Revengence cranks this element up to 11. Just about every character in that incredible game is a literal human weapon, created with invasive cybernetics that turns them into killing machines. The plot, bananas as it is, is a wonderful exploration and subversion of the trope.
The human weapon is often sad, but it serves as a launching point for a story about overcoming the expectations of others. A person built for war is an immense threat to others, but they're rarely enjoying the experience themselves. Sci-fi rarely stops at cool robot gun arms, the moral decisions that led there, and the questionable path that follows. A person reduced to a function, even if that function is murder, is in a constant struggle to attain true freedom. From historical antiquities to soaring superhero stories to unhinged cyberpunk chaos, the human weapon is a key aspect of modern sci-fi.