Prosecutors say the 22-year-old Kratz of Northeast Philadelphia helped his cousin, 22-year-old Cosmo DiNardo, kill three of the four young men in 2017.
Bucks County prosecutors have alleged that DiNardo lured the four victims, ages 19 to 22, to his family's Solebury farm under the guise of making marijuana deals.
The victims' bodies were found after a five-day search.
Three of them had been set afire and placed 12 feet deep in an oil tank converted into a pig roaster.
Kratz walked away from a plea deal earlier this year.
Bucks County Assistant District Attorney Kate Kohler's opening statement painted Kratz as a cold-blooded killer who was on a mission to "kill, rob, burn, and bury bodies" along with his cousin.
Kohler told jurors Wednesday that Kratz killed one of the victims and helped while Dinardo killed two others, then helped clean and cover up the evidence.
Kratz's attorney, Charles Peruto, painted a decidedly different picture of his client.
Calling Kratz an "idiot" and "slow" with a "tested IQ of 79," Peruto told the jury that his client is not a killer, but rather was too terrified and lacking the intellectual wherewithal to do anything that would have stopped the "lunatic" DiNardo.
He said Kratz was being "preyed upon by a psychopath."
Kohler, however, said there were multiple times Kratz could have called for help or even turned the gun on DiNardo. She called the slayings "just something fun to do that day because they could."
VIDEO: Cosmo DiNardo arrives at Bucks Co. prison
Peruto said his client was later manipulated into giving a recorded confession that is expected to be played in court. Both men are also expected to testify during the trial.
Peruto even went after Kratz's original court-appointed defender, Craig Penglase, referring to him in court as a "scumbag" for trying to reach a plea deal instead of trying the case in court.
The trial is expected to last one week and Kratz's attorney says his client will testify.
DiNardo and Kratz were charged with the July 7, 2017 slayings of Dean Finocchiaro, 19, of Middletown Township; Thomas Meo, 21, of Plumstead Township; and Mark Sturgis, 22, of Pennsburg, Montgomery County.
DiNardo pleaded guilty to the three killings and that of Jimi Patrick, 19, of Newtown Township, two days earlier. DiNardo is serving a life sentence.
DiNardo is expected to take the stand later this week.
The Associated Press contributed to this report.