Mr Collins, 53, was stabbed in the neck, throat and stomach.
A witness said the devoted father from Ireland desperately called out for help after he was attacked.
"He was yelling out 'help' and me and my mates we outside, we were freaking out and were like nah, we've got to help him," a man said.
"He was laying there and I was like mate calm down you will be all good, just keep calm."
Ms Pilapil - a registered aged care nurse - was allegedly slashed with a knife by her former partner and had to be hospitalised, the Sydney Morning Herald reports.
Witnesses said the terrified mother sprinted away and hid in a nearby shop before collapsing.
Villaluna was charged with murder and attempted murder after being arrested at the scene. Police apprehended him as he sat near Mr Collins' body, covered in blood.
A witness told the Sydney Morning Herald the alleged killer appeared "eerily calm", as he seemingly waited for police to arrive and arrest him.
Before the horrifying attack, both victims appeared to live happy lives - with loving photographs of their children adorning their social media accounts.
Mr Collins, who was the president of the Terrigal Wamberal Sharks Junior Rugby League Club, posted a recent photograph of him and his daughter, Aisling, earlier this year.
After he was killed, Aisling reposted the same photograph of her father, along with the caption: "I love you more than anything dad, I'll forever be your girl no matter what."
Ms Pilapil had a number of pictures of her young children littered on her Facebook page. Most recently, she shared a photo of a beautiful bouquet of flowers she had been sent by a "secret admirer".
The young mother's brother said he had spoken with his sister's children since the attack, the Daily Telegraph reports.
"They told me she was stabbed and she broke something and she was very lucky it did not hit her lungs and heart," Ralph Pilapil told the newspaper.
"We were very grateful. We were here in the church thanking that she wasn't badly injured."
It comes after it was revealed Ms Pilapil had taken out an AVO against her ex-husband just 51 days ago, according to Seven News.
The accused attacker was pictured on his knees with his hands covered in blood as police arrested him a short time later. A female police officer was pictured standing behind the man with her taser drawn.
A number of items, possibly including a bottle and knife, have been taken from the scene by police for forensic examination.
Another witness at the scene, Michael Short, said he saw the man walk past him with a hunting knife.
"Finished work at Hornsby only to come out in the mall to hundreds of people running and screaming and this bloke walking past me covered in blood holding a massive hunting knife," he said.
"Not knowing what to do I grabbed the tyre lever out of the trailer and followed him in case he tried to stab some on else. A few of us stood there hoping he would not stab any one else until the police came."
He claimed Westfield security did not show up until well after police had arrived.
"Not saying they had to arrest or stop him but someone had been stabbed. No first aid kit. Nothing," he said.
"I know they were there when it happened because the guard let me out of the door not a minute before all this happened."
- Daily Mail
