Some of the victim's relatives broke down in tears when Shappell entered the courtroom, his head lowered; others shook their heads.
Wagner said Shappell, who could have received a life sentence if convicted, took time to review the facts before accepting the plea.
"It's not something he decided quickly," Wagner told Senior Judge Thomas G. Gavin.
Carmody said the plot began when Morgan Mengel and Shappell began having an affair during the 2010 Memorial Day weekend.
"In less than three weeks, she's convinced him to kill her husband," Carmody said.
Carmody said that on June 17, 2010, Shappell spiked the victim's lemon Snapple at work with liquid nicotine, using poisoning instructions found on the Internet. When Mengel did not die quickly enough, Shappell, at the urging of text messages from Mengel's wife, bludgeoned him to death with two shovels, both of which broke, he said.
Morgan Mengel arrived to help clean up after Shappell texted her that the killing was complete, Carmody said. They stored the body in an unused garage bay at the West Goshen landscaping company, and Shappell buried the corpse near his alma mater, Marple Newtown High, on June 21, he said.
By June 25, the victim's family, frantic about Kevin Mengel's disappearance, had contacted police.
Shappell "panicked" when police arrived at the landscaping business and rode off in the victim's truck, Carmody said. Questioned by detectives, Morgan Mengel finally admitted the affair but pinned the murder plot on Shappell.
Police tracked Shappell to Colorado. He confessed to his part of the homicide "tearfully and immediately," but Morgan Mengel began working hard from behind bars to "keep pulling strings," Carmody said.
She sent Shappell letters from prison in which she fabricated a story that she had given birth to his twin boys in prison and that she needed Shappell to take the fall for the murder so she could raise them. She even provided a birth announcement with the infants' weight and eye color.
Carmody said that when Shappell learned from investigators that he'd been duped, "he was devastated."
Public records and interviews with people who know Shappell portrayed him as a quiet loner who endured a tumultuous home life. His father, Harry F., shot himself to death in October 2005 in the Newtown Square home where Shappell lived with his parents and a younger brother.
The suicide followed years of domestic strife and financial problems for Harry Shappell, an HVAC contractor.
Contact staff writer Kathleen Brady Shea at 610-696-3815, firstname.lastname@example.org, or @brandywinebits on Twitter. Read her blog, "Chester County Inbox," at www.philly.com/chescoinbox.