While I think pretty much all the characters in Game of Thrones were well-written, Viserys Targaryen was a character written to be a not a very likable person. He's a big jerk even to people important in his life. I think a lot of us forget he even existed but at the start of the show/books, he is the main contender from the Iron Throne against House Baratheon. He was the last remaining son of the Mad King Aerys II.
While Targaryens were well known for brother-sister marriages, the possibility of him marrying Daenerys was rather slim after being in exile with few friends, He needed to use her for political purposes which is what he did when he "gave" her to Khal Drogo. He gave Drogo one of the most beautiful princesses ever, and he was promised an army in return.
He doesn't take kindly to Daenerys having power, especially one greater than his own. But she spared him from the Dothraki because he was family.
Well that was until he threatened to kill her and her unborn child, and Khal Drogo killed him in response. Which paved the way for Daenerys to claim the Iron Throne for herself as she was the last known Targaryen in the world. He claimed to be a king but he was just a horrible prince.