Jeremy Steinke kept his head lowered as an audio tape was played at his trial Tuesday. (CBC)

An Alberta man being tried on three counts of first-degree murder admitted to an undercover officer and an inmate that he killed his girlfriend's parents for love, a jury heard Tuesday.

Jeremy Steinke, 25, is charged with killing the couple and their eight-year-old son in April 2006 in Medicine Hat, Alta.

In the second day of the trial in Calgary, the jury heard a 100-minute audio recording Tuesday of a conversation between Steinke and Const. Cory Both in which he's heard laughing, joking and bragging about the triple murder.

Steinke tells the undercover officer he tried talking his girlfriend, then 12, out of their plan to kill her parents, but he said that only made her more upset. He says he drank heavily and snorted cocaine that night before going to the girl's home.

"Because you love somebody enough, you’ll do anything for 'em no matter what the consequences," Steinke says to Both, who was posing as an inmate.

In the tape made on May 4, 2006, as he was being transferred in a police van from Medicine Hat to Calgary for a psychiatric assessment, Steinke recounts how he broke into the house while everyone was sleeping.

Medicine Hat police Const. Cory Both testified Tuesday about the conversation he had as an undercover officer with Jeremy Steinke in May 2006. (CBC)

He first stabbed the girl's mother, who had come downstairs, but then her screams brought her husband running down the stairs.

Steinke says the heavy-set man came at him with a screwdriver, cutting him in the eye. Steinke said the father's attack surprised him, but he managed to stab him, too.

"He came barrelling downstairs ... came at me real fast," Steinke is heard saying on the tape. "Last thing I really remember was him ... laying on the ground asking me 'Why?,' and I said ... your daughter ... she wanted it this way."

Steinke tells the officer that he then went upstairs with his girlfriend, where she slit her little brother's throat.

"It didn’t bother her at all either. She didn’t cry or anything. In fact, the next day when we were on the road ... she was laughing about it," recalls the accused.

'It was all her idea, but when someone you love asks you to do something, you do it.'—Alleged quote overheard by sheriff

The Crown is trying to prove that Steinke, plotted with his girlfriend to kill the family and then run away together because the girl's parents disapproved of their relationship.

The victims cannot be named to protect the identity of the girl.

Sounding much younger than his 23 years, Steinke seems pleased with the notoriety he has gained from the case, telling Both that he and his girlfriend "have become legends."

Steinke talks about plans to marry the girl in a gothic ceremony, get wedding ring tattoos and save up money to buy a 100-room castle in Germany.

Amid the conversation about the music, drugs and girlfriends, Steinke tells Both that he's always had dark thoughts, and admits to drinking blood.

Steinke kept his head lowered, and his gaze on the floor, as the jury listened to the recording.

Steinke told inmate he killed for love: sheriff

Earlier on Tuesday, Sean McGuigan, a former provincial sheriff based in Medicine Hat, testified that he overheard Steinke admit his role in the killings to another inmate by saying, "Yeah, she did the brother, and I did the mom and dad."

The inmate then asked if Steinke's girlfriend was crazy, to which he replied: "Yeah, she's f---ing crazy. I'm just a little bit crazy, but the drugs didn't help."

According to McGuigan, the inmate then asked whose idea it was, and the accused answered, "It was all her idea, but when someone you love asks you to do something, you do it."

The alleged conversation happened on May 2, 2006, as Steinke was being transferred from the courthouse to the remand centre in the southern Alberta city.

Steven Durk, another Medicine Hat sheriff, also testified Tuesday that he heard the same conversation.

The girl, who was found guilty last year of three counts of first-degree murder, is serving a maximum 10-year sentence for young offenders as part of a rarely used intensive rehabilitation program.

The trial is expected to last three weeks. If convicted, Steinke faces a life sentence with a minimum of 25 years in prison.

With files from Bryan Labby