AN AUTISTIC teen brutally murdered with an axe was protecting his stepfather from an ambush allegedly led by the man’s cousin, a court has heard.

The Supreme Court heard accused killer Darren Wilson, 33, told a police officer he was going to kill his cousin just days before 14-year-old Timothy “Timmy” O’Brien died from gruesome head injuries.

Senior crown prosecutor Andrew Tinney, SC, said Timmy became involved in the fracas after Wilson — disguised with a Betty Boop pillowcase — attacked his stepfather, Peter Williams, who had been lured to a Scarsdale property to be assaulted.

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“He was by the side of Peter Williams in the early hours of 5 January 2013, standing up for Mr Williams, and that was why Timothy met his death,” Mr Tinney said.

“Members of the jury, our experience of the present and our knowledge of the past inform us that the world can be a cruel and vicious place at times. “But even acknowledging all of that, the very idea of two men killing a 14-year-old autistic boy by hitting him on the head repeatedly with an axe is almost incomprehensible.” The court heard Wilson launched a surprise attack on Mr Williams and began repeatedly punching him in the face.

Mr Tinney said Timmy jumped to Mr Williams’ defence, belting Wilson and his co-accused with the bat, injuring and angering them both.

The co-accused, who cannot be named, allegedly chased Timmy outside and repeatedly hit his head with the blunt side of the axe and said: “Die, c---, die.” Wilson then allegedly punched Timmy to the head repeatedly until he appeared to lose consciousness.

“The accused then picked up the axe … and then using the blade of the axe, the sharp side of the blade, the accused repeatedly struck Timmy to the head, and to the body, with the axe,” Mr Tinney said.

He said Ms Trezise was hit and threatened when she tried to pull Wilson off the boy.

The prosecutor said Wilson delivered the fatal blows with the axe or, alternatively, acted as part of a joint criminal enterprise. During his opening address to the jury, defence counsel Tom Danos said Wilson was knocked out by Timmy’s blows with the bat and his co-accused had acted alone in the “callous and brutal attack, totally unjustified and in circumstances in which (Timmy) was clearly an innocent victim”. Mr Danos said Wilson’s co-accused “went off on a frolic of his own” and there was nothing to suggest his client “assisted in any shape of form in the attack on poor Timmy O’Brien”.

The court heard Wilson harboured great animosity towards Mr Williams and was no longer welcome at his home after leaving him a threatening note. Wilson allegedly made plans with a young Scarsdale woman for her to lure his cousin to the property by claiming she was scared of prowlers, so Wilson could attack him.

Driven by his associate Lisa Trezise, the Ballarat man picked up his co-accused — who got into the car armed with an axe and a gothic-style knife — and went to the property, the court heard.

Mr Tinney said Timmy, who was “almost inseparable” from his stepfather, armed himself with a baseball bat and joined his stepfather in attending the property despite the women requesting Mr William’s assistance around 4am.

“That's how close they were,” the prosecutor said.

“And events transcribed that loyalty and that closeness were to cost Timothy O'Brien his life within a short time of leaving his home.” The trial before Justice Betty King is expected to last six weeks.

emily.portelli@news.com.au