Kratz is charged with criminal homicide, conspiracy, abuse of a corpse and other charges in the 2017 deaths of four friends in Bucks County.
Prosecutors say Kratz helped his cousin, Cosmo DiNardo, kill three of the four men. Their bodies were later found on DiNardo's family farm in Solebury, Pa.
DiNardo took a plea deal last year for the murders of Dean Finocchiaro, Thomas Meo, Mark Sturgis and Jimi Taro Patrick. Kratz is charged with first-degree murder in the deaths of Finocchiaro, Meo and Sturgis.
DiNardo pleaded guilty and is serving a life sentence. Kratz walked away from a plea deal earlier this year.
In court on Friday, prosecutors played hours of video of his interrogation with police days after the murder.
The jury listened to Kratz on police interrogation video, as he denied any involvement in the murders. He said he didn't report the crimes to police because he was afraid of DiNardo.
He continued to paint DiNardo as a bad person and at one point, when he was alone on in the interrogation room, said: "What an evil (expletive) kid. I hate you, you sick monster."
His mom was allowed in the interrogation room. She encouraged him to tell police everything and urged him to tell the truth.
She described DiNardo as a psychopath and told Kratz, "Save yourself because he will bury you. You can't bring them back but you can bring them justice."
On the interrogation tape police continued to say Kratz wasn't telling the truth. After speaking to his mother, Kratz eventually told detectives hat DiNardo hid two guns on his aunt's property.
Kratz's attorney claims his client has a low IQ and was just too terrified to do anything that would have stopped DiNardo.
Kratz could face the death a penalty if convicted. Both he and DiNardo are expected to testify during the trial.