In Celosa's mind Mary didn't deserve her position. Where Celosa came from people like Mary didn't rise through the ranks. To Celosa, it seemed unfair. Although Celosa had no education to first and a resume that didn't compare to even half a bullet point of Mary's, Celosa thought, "How could it be that I have to do such menial tasks and Mary gets paid for what appears to be not working at all." Celosa felt that she deserved more and that if she succeeded in destroying Mary by turning others against her and getting her fired that she would have more happiness and success. Celosa foolishly felt that if Mary were to go away that she would finally get the respect that she deserved.
Now Celosa had been at the firm much longer than Mary. The management team had Celosa doing their dirty work literally and figuratively since she got there. Clearly, Celosa was illogical and had misplaced anger. She was a gossip, an instigator, a stalker, a trash collector, a janitor, a maid, a cook, a handyman and errand boy and not much more. Celosa admirably did whatever was needed to be done whether it was morally sound or not.
Mary knew Celosa had a hard home life. She had less than a decade to retirement, was the sole bread winner and was willing to do anything to keep a job including destroying another. Celosa made it clear to Mary how she felt but Mary refuesed to treat or look at her the same in return. Mary didn't believe in repaying evil with evil. Mary felt sorry for Celosa. She pitied her.