If we ignore the monstrosity that was Deadpool in his X-Men Origins: Wolverine cameo, the new fan favorite actually has one of the most interesting backstories of any Marvel character. In fact, even though not much of it gets talked about in his first film, he was actually a contract killer since his teenage years.
Wade Wilson, aka Deadpool, had always been a badass long before he became an adult. He grew up without a mother and with an abusive father, who was killed by one of his friends. Wilson was a teenage criminal until he joined the military, traveled the world and subsequently became a contract killer.
Deadpool’s appeal actually comes from the fact that he only kills people he deems worthy of a painful dead. He never takes a contract against an innocent man, and in most occasions his primary targets are mob bosses and criminals.
You might’ve also read (or heard) that Deadpool and Spider-Man boast a “bromantic” relationship that goes back many years. Although this is highly unlikely to get presented in the movies due to Disney and Fox owning both character’s separately, the comic universe presents a fantastic story that goes back many years indeed.
Not many people actually know this, but Wade Wilson’s first encounter with Peter Parker was during a time travel accident. Deadpool got sent back to the past and ended up contacting Spider-Man and getting him to move on, as Wilson takes Parker’s identity to substitute him in a fight against Harry and Norman Osborne.
Interestingly enough, according to Deadpool’s original story, he infiltrated the Yakuza before falling in love with his movie darling, Vanessa. His criminal background isn’t fully explored in Fox’s films as it rather focuses on the comedic side of his antics, but there’s definitely a lot more to learn about the renegade hero.
If you loved the movies, you should definitely check out the comic books!