On one occasion, a few weeks before the alleged murder, there was a dispute about Merritt refusing to take rubbish from a bank he had cleaned.
"I saw him that night and he was pretty livid about that. I believe on that occasion he said he'd like to burn her family alive in front of her," Mr Kelsall said.
"I told him I thought it was an overreaction. He told me that it wasn't and she would deserve it . . . he was very very angry."
The witness also said he heard frequently heard Merritt comment on their supervisor's work ethic.
"He would say: 'the lazy bitch, it would be fine if she died'," Mr Kelsall said.
"It was fairly regular; definitely more than five times I would say."
When everyone turned up for work just hours after Ms Ross's death on the evening of December 2, Mr Kelsall said he noticed scratches on Merritt's face and a black eye.
Another colleague asked the defendant what had happened and he told them he had scratched himself in his sleep.
He then grabbed his car keys and walked outside.
Mr Kelsall told the jury he pursued the 21-year-old to his car and struck up a conversation.
Eventually he asked Merritt how he felt about Ms Ross's death.
"I don't care. You know how I felt about her," the defendant allegedly told him.
The trial is scheduled to run for another week.
