From The TV IV
Wayne Janssen had an affair with Diane while she was married to Sam Austen. While Sam was away for work, Wayne got Diane pregnant, but Sam returned and worked things out with Diane. Kate was born and Sam loved her as his own child, and Wayne never knew that he was her real father.
Wayne and Diane continued their affair and eventually Diane decided to leave Sam so that she and Wayne could get married. However, Wayne was a terrible husband to Diane, getting drunk night after night and beating Diane up. Kate grew up seeing how badly her "stepfather" treated her mother, and she hated Wayne for it.
Although Wayne never abused Kate, he would often make sexual comments to her while he was drunk. One day, Kate discovered photos of Sam in Korea up to four months before she was born. She realized the horrible truth that Wayne was in actual fact her biological father. She couldn't stand being part of him and sharing his flesh and blood, so she decided to kill him and to rid her mother of his abuse once and for all.
Kate devised a plan and waited for Wayne to return from the bar one night. She helped him into the house, put him into bed and put up with his comments for one last time before she left and blew the entire house up, killing Wayne inside and making it look like a gas leak.