The defense is attempting to show the mentally ill homeless man had a nearly two-decade history of violence, and that he contributed to his death by resisting arrest.
Thomas' 91-year-old grandfather, Walter Dieball, and mother, Cathy Thomas, were called to testify. Defense attorney John Barnett has said Thomas attacked his grandfather with a fireplace poker in 1995.
"I turned around and started watching TV, and a few minutes later, I heard a rattling of the fireplace tools. I turned around and he had a poker in his hand and he hit me with it," said Dieball.
Thomas was later charged with assault with a deadly weapon. Dieball says the altercation back then has nothing to do now with his grandson's murder case.
"I think they are damn guilty. Nobody should have to be beaten to death," said Dieball.
But the defense hopes to show jurors Thomas was no stranger to violent run-ins that he caused. One witness recounted how he was punched in the face when he asked Thomas to leave a party. Another said Thomas threw rocks and tried to hit her with a metal bar when she refused to let him use a bathroom.
Thomas' mother also reluctantly testified that her son grabbed her by the throat and held on for several minutes in 2010.
"He had schizophrenia. He wasn't in his right mind when all that happened," Cathy Thomas said.
Thomas died in July 2011 after a violent encounter with six police officers at a Fullerton bus depot. The encounter was caught on surveillance video. Former Officer Manuel Ramos, who is seen beating Thomas, is charged with second-degree murder. Former Officer Jay Cicinelli faces involuntary manslaughter. Both men have pleaded not guilty.
Another officer will be tried separately on a charge of involuntary manslaughter. Three other officers were not charged.
Prosecutors rested their case Wednesday. A surgeon testified Tuesday, saying Thomas arrived at the hospital in a comatose state with failing organs.
Dr. Michael Lekawa, chief of trauma surgery at UC Irvine Medical Center, treated Thomas and said he never regained consciousness.
Lekawa also said the cause of death was lack of oxygen to Thomas' brain he suffered during the struggle with officers. Under cross-examination, Lekawa acknowledged that Thomas's broken ribs could have been caused by CPR.
The Associated Press contributed to this report.