Sebastian Kalinowski, 15, died as a result of complications from untreated rib fractures caused by his stepfather, martial artist and steroid user Andrzej Latoszewski, 38.

Agnieszka Kalinowska, Sebastian’s mother, encouraged Latoszewski’s abuse while also participating in the assaults at their home in Leeds Road, Huddersfield.

During nearly five weeks of testimony, the jury at Leeds Crown Court was shown harrowing footage of Sebastian enduring horrendous treatment from CCTV cameras installed in the couple’s own home. According to the prosecution, the cameras were installed in part to spy on Sebastian.

Bodybuilder Latoszewski, who practises Thai boxing, repeatedly kicked, punched, and stamped on the adolescent. He was also beaten with weapons such as a stair spindle and a bed slat. A flex cable was used to whip him.

Sebastian, who moved to the UK from Poland in October 2020, was so conditioned to his own mistreatment, according to the prosecution, that he would sometimes hand the weapons to his abusers before being attacked.

Sebastian’s mother participated in the assaults and the constant verbal abuse he received, and she held Sebastian down while Latoszewski beat him. Messages between her and Latoszewski showed that they were encouraging one another.

He was also subjected to humiliating exercise drills such as hundreds of push-ups and squats. He was seen being forced to do chores and spending long periods of time alone in his bedroom.

The prosecution claimed he was used as a “human punchbag” and that his treatment was “torture.”

Even the defendants’ own lawyers admitted that the case was one of the worst the courts had ever seen.

Before the jury retired to deliberate, Latoszewski’s lawyer, Nicholas Lumley QC, said, “The treatment of Sebastian is truly shocking, and it should not happen in a civilised, or any, society.”

No matter how hardened we are to what happens in criminal courts up and down the country, this case really takes the biscuit,” Kalinowska’s QC Leonard Smith told the jury.

Sebastian collapsed and lost consciousness after being assaulted on August 13, last year, according to the court. But it took Latoszewski more than two hours to call an ambulance.

Meanwhile, the couple was dressing Sebastian’s body. According to the prosecution, Latoszewski was seen putting water into Sebastian’s mouth to make it appear as if Sebastian had drowned.

He then took down the CCTV cameras.

Sebastian was discovered with over 80 injuries, including 23 untreated rib fractures. The injuries resulted in infection, which proved fatal.

Medical personnel were stunned as Latoszewski laughed and joked about Sebastian being “naughty” and that “the most he would ever get was a smack.”

Both Latoszewski and Kalinowska were charged with murder, as well as manslaughter, causing or allowing a child’s death, and child cruelty.

They initially denied all charges, despite overwhelming evidence. Latoszewski pleaded guilty to manslaughter after the trial began, while Kalinowska admitted child cruelty. They maintained their denial of murder, claiming they had never intended to kill Sebastian.

The couple claimed that their abuse of Sebastian was “reasonable punishment” for alleged misbehaviour, such as lying and skipping school.

During the trial, Kalinowska, who claimed she was under Latoszewski’s control and acted as his “puppet,” was asked what she was thinking when she hit her son.

“That I would just hit him like a mother would a son,” she explained. “I thought it was a normal parental punishment, that if a mother hits her son on the head, it’s a normal thing,” she added.

When asked about one particular assault, Latoszewski explained, “It was because he had been lying for a long time and we didn’t know what forms of punishment to use because the normal form of punishment had failed to work, and I just wanted to show him something more aggressive during school holidays so he would listen.” It’s not an explanation, but it’s all because of inexperience.

Latoszewski claimed he assaulted Sebastian on the day he died because the boy took “too long to get dressed.”

According to the court, there was no evidence to back up the defendants’ claims that Sebastian misbehaved. In fact, North Huddersfield Trust School, where he was a student, described him as a “model student.

The jury reached unanimous guilty verdicts in less than three and a half hours.

The sentences for Latoszewski and Kalinowska will be handed down at a later date.