A West Australian man has been found guilty of sexually assaulting and murdering a 10-month-old baby in his care in March last year.

For 15 hours, Mervyn Kenneth Douglas Bell tortured a 10-month-old baby, inflicting broken bones, burns and bruises on the boy who was on the verge of learning to walk.

His motive? Payback for the child’s mother humiliating Bell the night before when they fought at a house and he left her bashed, naked body lying on the ground.

After the mother was hospitalised and arrested by police, Bell took the child from his Broome family in March last year and drove almost 1000 kilometres on a very hot day.

He eventually took the injured boy to the Fortescue Roadhouse near Karratha – but it was too late.

On Friday, West Australian Supreme Court Justice John McKechnie found Bell, 27, guilty of sexually assaulting and murdering the boy.

Justice McKechnie, who presided over Bell’s trial without a jury due to the graphic details of the evidence, said it was one of the most evil cases he had ever encountered.

He rejected Bell’s claims the child’s injuries were caused when he fell out of a moving car and when he sat on hot rocks.

“I have a low opinion of the accused’s credibility. He is quite prepared to lie when it suits him,” Justice McKechnie said.

He said Bell was the only person who could have inflicted the injuries and had done so deliberately.

Justice McKechnie said while Bell had not intended to kill the baby, his overall intention was to injure the child enough to threaten his life to get back at the boy’s mother, and had failed to provide him with the necessities of life.

“The deceased needed urgent medical attention and the accused conspicuously failed to provide it, probably because he knew the trouble he would be in,” he said.

Bell showed a “callous disregard for his health and safety”, Justice McKechnie said.

He warned Bell he would face a life jail term when he is sentenced in December and indicated he might consider imposing an indefinite prison sentence, pending the results of psychological and psychiatric reports.

The boy’s mother was hospitalised after Bell attacked her but was also charged over an assault on police, the outcome of which is pending.

She and her supporters wept and cheered in court when the judgement was handed down.

“But nothing will bring my son back,” she told reporters outside court.

The mother also demanded answers as to why police did not search for her baby immediately when her father reported his abduction.

“This could have all been prevented and I still might have my son today,” she said.

Police told AAP the Kimberley District Office had begun an internal investigation.

“The Corruption and Crime Commission and Police Internal Affairs has oversight of this investigation, which was referred to WA Police by the CCC in May 2013,” police said.

The family said the loss of the baby was an “unbearable grief” and hoped Bell would never be released from prison.