The letter he received from Spotless outlining the concerns and inviting him to a meeting with bosses contained information the process may result in dismissal.
That sent Merritt into a murderous rage, prosecutor Robin Bates said.
The court had heard in the first week of the trial he had told a colleague he wanted to burn Ross's family alive in front of her, as their working relationship deteriorated.
Only hours after receiving the disciplinary letter, knowing his supervisor would be finishing her shift, Merritt drove back to work.
Armed with a hammer, he attacked her from behind as she loaded rubbish into a skip.
Blood on the inside of the victim's white van showed she had tried to escape, the Crown said.
A bloody hand print on the side of the vehicle had occurred when Merritt dragged Ross out of the seat and continued the attack.
Blood spatter on the van and the ground showed the brutal assault continued in more than one area but the victim eventually succumbed to more than a dozen blows to her head.
Bates said blood markings indicated at least one of the wounds was inflicted while Ross was lying on the ground.
There could be no confusion about Merrit's intent, the Crown said.
A bloodied hammer, hoodie and glove were found in a wheelie bin outside the defendant's home less than a week after Ross's death, after police enquiries led them to the Nairn St property.
Forensic scientists gave evidence that testing indicated DNA on the top was "extremely" likely to have come from both the victim and Merritt.
There was also DNA 430 times more likely to have come from Merritt than anyone else that was found on or under the finger nails of the dead woman.
Merritt showed up to work the next day with scratches on his face and one workmate said he was flushed and grinning.
He told colleagues, family and the police the scratches were self-inflicted in his sleep but that did not fly with the jury.
The Crown said it was an attempt to explain the inexplicable by the defendant.
