CALGARY - A man accused of killing his girlfriend's family admits he stabbed her parents when they surprised him as he lurked in their basement waiting to meet their pre-teen daughter.

But Jeremy Steinke testified Monday that it was the 12-year-old girl who slit her brother's throat.

Steinke said he climbed through the basement window of the family's home in Medicine Hat, Alta., after making plans to meet with the girl, who had been grounded for trying to spend time with him.

Steinke, who was 23 at the time, is charged with first-degree murder in the deaths of the parents and the eight-year-old boy.

Jumpy and high on drugs and alcohol, Steinke waited for her in the darkness, he testified. He said he heard footsteps and turned the corner expecting to see the girl.

"That's when (the mother) turned on the light, seen me and screamed," he told the jury. "I freaked out and I ended up pulling my knife out of my hoodie pocket and inflicted a wound on (the mother)."

Steinke said he turned away from the woman's bloody body to see the girl's father charging down the stairs after stopping to grab a screwdriver. Steinke said there was a brief standoff before he tried to back away and tripped.

The father jumped on him and the men struggled, he said. The father punched or jabbed him in the eye before he could struggle free, Steinke recalled, adding he then tried to run away.

"But you stabbed (the father)," pointed out defence lawyer Alain Hepner.

"When he attacked me with the screwdriver, yes, I did."

With both parents lying behind him in a pool of blood and with the walls and floors of the room spattered red, Steinke went upstairs to find his girlfriend. He said she met him in the kitchen and the couple shared a kiss.

The girl then turned around and climbed some steps to the home's next level, while he waited in the kitchen for a little while before following. He said he could hear a conversation, but couldn't make out what was being said.

"That's when I looked in (the boy's) room and seen her cut his throat," he testified, his voice growing unsteady. "He was lying on his back on the bed and she was standing over him."

Steinke insisted repeatedly that he didn't touch the boy.

He said the girl embraced him again before he told her he was going outside to get some air. He said he began panicking again, jumped in his truck and drove away, stopping only to vomit on the roadside before heading home to shower.

"Did you go over there with the intention of killing (the parents)?" asked Hepner.

"No I did not," answered Steinke, who also told court he is now on medication for depression and paranoid schizophrenia. "I went over there with the intention to pick (the girl) up and return to my residence."

Hepner opened his defence of Steinke by saying his client's crime was not premeditated -- a condition that would be necessary for a first-degree murder conviction.

Steinke said he only remembers stabbing the woman once, although the court has heard expert testimony that she was stabbed 12 times. He said he couldn't remember how many times he stabbed the father, who the medical examiner said was stabbed 24 times.

"I didn't know what was going on. I was just panicking, freaking out."

He testified he almost always carried a knife after he was once jumped and beaten up. He added he sometimes carried a ninja sword strapped to his backpack.

The girl has already been convicted of first-degree murder and is serving a youth sentence. She cannot be identified under the Youth Criminal Justice Act.

Steinke said he thought she was 16 for the majority of their relationship and she told him she was 13 not long before the deaths.

Court has heard evidence from several witnesses who testified Steinke told them he planned to kill the family and wanted help. Other witnesses said he talked about the crime while watching the movie "Natural Born Killers," saying it would be the same except that the boy would die.

Steinke said he never asked for help from anyone and never said a word while he watched the movie.

Asked about Internet messages he wrote to the girl about a plan to kill her family, he said he and his friends always discussed killing people who annoyed them and it had no meaning.

Violent song lyrics about death posted to his Internet profile simply mimicked the Goth and death metal music he loved, he said.

He also said he wrote about things that he would never be able to do in real life.

"I was just expressing something I couldn't do physically, that I wouldn't do physically."

Court has heard that the day after the bodies were discovered, Steinke and the girl were arrested after police found them in the box of a truck near Leader, Sask.

Steinke confirmed that he and the girl spent the next day together, but he said they never discussed what happened. He said they were in the truck because they tagged along while a friend was being given a ride home.