Vile Facebook messages were sent to the ex-girlfriend of a 15-year-old boy shot dead in a suspected revenge attack in Sydney, Daily Mail Australia can reveal.

A string of taunting messages sent to slain Brayden Dillon's former partner say his death was "revenge" and that he "should have seen this coming".

A teenage girl who owns the account that sent the messages told Daily Mail Australia she had been hacked and claimed she did not send the sickening messages.

The messages were sent just hours after Brayden was fatally shot by a masked intruder who had broken into his bedroom at his family home in Glenfield.

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The Facebook messages say: "He should of seen this coming [sic].

"It's called revenge."

They continue: "And braydens a mutt. And he deserved everything his just got. Hahahahh. He's gone now. Can't bring back the dead hahah [sic]."

In a later message, the user says "we all knew this was going to happen".

"Maybe you didn't but I did, and I know who shot him as well. Been planned for 6 months."

The girl who owns the Facebook profile insisted that her Facebook had been "hacked" and said one of her friends also knew the password for her account.

She later wrote in a public post: "I'm sorry for what was said and its 100% wrong, my Facebook's hacked, honestly I don't know when the f***s on my account but I changed my password [sic]."

Police have refused to comment on whether Brayden's shooting was a "revenge attack" linked to another fatal attack.

Brayden was shot at point-blank rage as he lay in his own bed this morning after a man broke into his family home on Moresby Street in Glenfield.

He was taken to hospital in an "extremely critical" condition but died shortly after 11am (1pm NZ time), police said.

A man broke into the boy's family home shortly after 6am, sneaking into the boy's bedroom and opening fire "execution-style".

After shooting the boy in the head at close range, the intruder - whose face was covered - escaped back out of the front door.

The teenager - who was seen being loaded into an ambulance with his face drenched in blood - died at The Children's Hospital in Westmead, Sydney.

His mother, stepfather and two siblings, aged seven and one, later left the home with a police escort.

Detective Chief Inspector Mark Brett said earlier that shooter entered the boy's home and then "forced entry to the victim's room".

"The mother is extremely distressed. She's at hospital with her son," he added.

Police are now looking for a man described as being aged between 18 and 20, with a thin build, who was last seen wearing grey tracksuit pants, a navy blue hooded jacket and white joggers.

It is believed that he may be armed with a pistol and the public have been warned not to approach him but to immediately call triple-zero.