He was a South African who fought in one of their wars against the British. He was captured and, knowing wang can solve any problem, seduced the daughter of one of the guards to gain freedom. He then made his way to England where he convinced them to let him join the army and got stationed back at South Africa, with the intention of thoroughly fucking things up from the inside. Keep in mind, he wasn't spying on anyone's behalf at this point. He was working as a saboteur just to be fucking doing it.
Already pissed at the British for locking him up, his hate rose to a boiling point when he returned to his parent's farm in South Africa and realized the British Army had killed his sister, sent his mother to a British concentration camp and the entire place was littered with unsold Robbie Williams' CDs ensuring nothing would ever grow on the land again.
Cracked Fact: Robbie Williams was in English boy band "Take That" and therefore deserves endless mockery.
As revenge, Duquesne planned to completely destroy Cape Town, where Lord Kitchener, the man responsible for what happened to his family, was staying. He recruited 20 men to begin his tour of destruction, but was betrayed by the wife of one of his cohorts and wound up imprisoned again. He managed to escape the death penalty by giving away secret South African codes (they were bullshit), receiving life imprisonment instead while his 20 men were forced to eat English food and then shot.
After the war, they made the mistake of releasing Duquesne. Still holding a grudge over the whole "slaughtered sister" thing, he headed to Scotland, tracked down Kitchener and sabotaged his boat, killing him.
Moral: Don't fuck with Duquesne, he'll find you eventually.
Wait, we're not done. High on life but low on people to kill and now a very wanted man, he faked his death by writing his own obituary in the New York Times and told the world he'd been killed by legions of Amazonian natives wanting to steal his treasure (his first choice was saying he'd wrestled 30 tigers and then was sexed to death by 50 pornstars from the future, but it had been used by a different madman a week earlier). Duquesne then disappeared.