He had a long history of violence and alcohol abuse. He sexually and physically abused Angela throughout her entire life, and possibly his wife as well. After Angela finished high school, she ran away from home in an effort to escape her abusive lifestyle, but was found and forcibly brought back by her father.
Far from innocent, her mother was aware of the abuse and lead Angela to believe that she deserved such treatment. It is implied that Angela's brother, learning from his depraved progenitor, also abused his sister - the combined trauma of which eventually drove her to kill them both.
A newspaper article found in the labyrinth reveals that Thomas was murdered around midnight. Angela achieved this by stabbing him multiple times in his neck and left side of torso in a fit of rage with a knife. There is evidence of a struggle in the room, implying that Thomas fought for his life, but was overwhelmed by Angela. She did not hide her father's corpse and fled the house. Police eventually found his body and since there was evidence of a homicide, they opened a murder investigation.
A photo left in Room 109 of Blue Creek Apartments in which Angela was residing shows a mother, daughter, father and son in that order. The photo is torn in half between the father and daughter. This could have been done by Angela, showing that the abuse happened when she was young. It is possible that he is Thomas.
In the creased photo, the father appears to look young himself, wearing a checkered shirt. He has what looks to be a five o'clock shadow and short brown hair, while his eye color is not clear. It is possible that the creased form of the photo symbolizes Angela's view on her family as "broken".