He overreacted...
The morning after the night before, she explained alcohol made her use poor judgement, she realised it was a mistake...and he still turned all cold on her after three years of dedicated service and friendship.
And Nora used her stuff and belittled her about her feelings for the boss...
Her workmates bitched about her and even went to the boss with their theories...none of them proven...that got her fired...
Apart from murdering the wife, Rachel was a normal nut surrounded by very nasty nuts...any wonder she cracked.
I'm a Lieutenant Detective in, uh, Homicide. That's a fancy name for murder.