Sawyer is a troubled character, he's the guy that everybody hates, he's sarcastic, selfish, and can be irritated easyly. He loves merschandising, he steals and hordes whatever he can. he was a conman before he crashed.
He watched his parents die in murder/suicide after being swindled out of their life savings by a con man named "Sawyer".He's obsessed with hunting down the original "Sawyer" for revenge; holds him responsible for his parents' deaths and he wrote a letter. He become a con man himself and adopted Sawyer's name He doesn't like what he became and to be hated convince himself that he's a bad guy.
He killed the wrong man thinking he was "Sawyer".
One of his cons, Cassidy, figured out what he was doing and asked to be a part of it. The two conned together for awhile until he conned her. He ended up in prison, where Cassidy visited him to tell him he has a daughter. Sawyer made a deal with the warden and was let out of prison. He anonymously sent his reward money to Cassidy's daughter.
On the island, he's a solitary boy, he stubbornly allowed Sayid to torture him to prove a point. he loves reading and criticizing everybody, he's not a such good compagny! As Kate said, he wants to be hated.
In spite of his appearance of "bad boy", Sawyer has a heart buried under his wickedness...Kate is the only one to make him love himself, he loves her even if he doesn't want to admit.He's afraid of what his feelings could engender at him.