Defence lawyer Alain Hepner argued Wednesday that Jeremy Steinke, shown in this undated photo, should not be convicted of first-degree murder. (CBC)

Jeremy Steinke killed his girlfriend's parents on an impulse, but there had been no plan to harm them or their eight-year-old son, his defence lawyer argued Wednesday.

In his 45-minute closing argument, Alain Hepner told the Calgary jury that his client killed the parents, who objected to Steinke dating their 12-year-old daughter. But he said Steinke did not plan the attacks and did not touch the boy.

He referred to Steinke's earlier testimony that it was the girl who slit her brother's throat.

Steinke should be acquitted in the death of the boy and is, at most, guilty of second-degree murder or manslaughter of the parents, Hepner argued Wednesday afternoon.

The family's bloody bodies were found in their Medicine Hat home in April 2006. The victims cannot be named to protect the identity of the underage girl, who is serving a 10-year youth sentence.

Steinke testified this week that he killed the parents because he panicked. He said he had entered the home through a basement window and was waiting to meet the girl to sneak out when the mother discovered him and screamed.

Hepner said Steinke "snapped" and "impulsively" killed her, as well as her husband, who came running down the stairs.

Because of a lack of premeditation, the jury should not convict his client on first-degree murder charges, Hepner argued.

Forensic evidence damning, says Crown

The Crown is arguing that the stabbings were premeditated by Steinke and his girlfriend as part of a plan to kill her parents and run away together.

Crown lawyer Ramona Robins, seen in a court sketch, told the jury Wednesday that forensic evidence in the case is enough to convict Jeremy Steinke. (CBC)

Crown prosecutor Ramona Robins said her case proves Steinke is guilty of first-degree murder beyond a reasonable doubt, pointing to e-mails he exchanged with his girlfriend before the killings, and forensic evidence that placed him at the crime scene.

Blood found on Steinke's clothes and shoes shows he was present for all three slayings, she argued in her closing statement.

Robins dismissed a defence argument that Steinke was too drunk and high to plan the killings, emphasizing that he was sober enough to remember stopping at a store to buy gum and what route he took home on the night of the fatal attack.

Testimony from aimless not significant: defence

The e-mails that outlined killing the girl's parents were just talk typical of people immersed in the goth subculture, alleged Hepner.

The black neoprene mask Steinke wore, and the knife he carried, on the night of the killings were also part of the dark style of costume favoured by goth followers, said the lawyer.

Hepner also asked the jury not to give too much weight to testimony from Steinke's friends, calling them kids and aimless souls who were usually drunk, stoned, or both.

A taped conversation between Steinke and an undercover officer, in which the accused is heard bragging about the killings, was full of "half-truths," said Hepner, because Steinke was living in a fantasy world.

The case is expected to go to the six-man, six-woman jury after the judge's final instructions on Thursday.

With files from Bryan Labby, Scott Dippel