KAMLOOPS, B.C. - A B.C. father who admitted to killing his three children testified at his first-degree murder trial Thursday that he believed the children's mother cheated on him repeatedly and he questioned whether their youngest child was even his.

Allan Schoenborn is on trial for three counts of first-degree murder in the deaths of 10-year-old Kaitlynne, eight-year-old Max and five-year-old Cordon.

He has recounted the killings in his testimony in B.C. Supreme Court, saying he believed they were being molested and killed them to protect them from lives of drugs and sexual abuse.

Defence lawyers are arguing that the killings were the result of mental illness, and on Wednesday painted Schoenborn as a loving but extremely paranoid father who believed the children were being abused.

Prosecutors, however, contend the killings were an act of vengeance against the children's mother, from whom Schoenborn was separated.

While mostly calm and attentive in the witness box, at times during his cross-examination Schoenborn raised his voice and cursed as Crown lawyer Glenn Kelt led him through a variety of scenarios, asking him how he felt at the time.

Kelt questioned him at length about a phone call he made to police shortly before the slayings to report that his daughter was being "groomed for a life of prostitution."

Schoenborn became increasingly loud and insistent in his response, saying he didn't use those words with the officer, but was certain the man understood.

"I was being clear to the officer, I was being very clear. Listen to me, he sloughed me," he answered and then swore.

Schoenborn, 41, said he felt he reached out for help many times, but always met a brick wall.

He recounted several incidents where he accused the children's mother, Darcie Clarke, of cheating on him, including with her own brother. He said she always denied the accusations and reassured him.

Asked about the allegations of infidelity, Schoenborn said he felt angry, confused and certain she was lying to him.

"I feel angry, yeah, I feel angry because I can't get to a truth," he said.

The judge, who is hearing the case without a jury, heard that the Ministry of Children ordered the parents to spend time with the children separately - an order that angered Schoenborn.

"No, I was not angry, I was a little headstrong at that office (where the ruling was made) and also determined," he told the court.

Schoenborn told Justice Robert Powers, who is hearing the case without a jury, that he had in the past doubted that he was actually the father of his youngest son.

He explained that while his two older children bore a remarkable physical likeness, his third wasn't as close.

"It's harder to accept Cordon as my son," he said.

Schoenborn testified that in the 15 years he and Clarke were together he sometimes feared she was being "oppressed."

"I think she's being told what to do and how to act...by the drug dealers, by the scum."

Two psychiatrists have observed Schoenborn's testimony and will provide expert opinion on his mental state later in the trial.

The children's bodies were found by their mother in their Merritt, B.C., home on April 6, 2008.

Schoenborn recounted in his testimony Wednesday how he swung and slashed Kaitlynne to death with a cleaver, and suffocated the two boys.

He has testified that he believed the children were being sexually abused, and he felt he had to kill them rather than let them continue to suffer.