Their parents sound sociopathic, so that's a bad start. They were both abandoned, with mental, physical, and probably sexual abuse in between periods of abandonment. While the eldest was raped, it wouldn't surprise me if the younger two were as well.
Then they get into a situation where they're put through mental abuse by their employer. It's not a shock that the pressure became too much and they exploded.
I recommend Dr. Michael H. Stone's "The Anatomy of Evil" (and the follow-up "The New Evil"), if you've not read them. (https://smile.amazon.com/Anatomy-Evil-Michael-H-Stone/dp/1633883353) His Gradations of Evil scale is pretty interesting.
If you are interested in Murders i can recommend "The Anatomy of Evil" from Dr.Stone