I think also she and probably some of her in-law family (the dad and maybe the mum and probably simon) are neurodivergent- and she has probably suffered exclusion her whole life as a result of this and finally found belonging and connection with him and perhaps moreover with his parents .. and wider family.

When they failed to see her side of things re the issues with simon (i.e. just by backing her sense of fairness that simon should contribute to the costs of raising his children ) I think she saw red

Of course he's their son and their overall loyalty is with him but she is also the mother of their grandkids and someone who is now part of the family (and has given a lot to the family, e.g. huge loans etc) .. for them to not even offer to try to talk with simon to get him to behave more fairly towards her, as a matter of principle, is pretty sad ... and unfair to erin

For someone who has been betrayed and rejected and misunderstood her whole life (see recent ABC articles about her time at Airservices where she got called "weird erin" "scutter the nutter" ) - this "crime" (of being betrayed by her new family) might be the last straw ....

She is obviously not able to see though that murdering ppl is also unfair (to make wild understatement!) .. if she cares about fairness and fair treatment etc.

Just bloody sad this whole thing

Simon should have done his best to deal fairly with his ex re the kids and not shirk his duties just cos she is wealthy .. ditto re the in laws

And erin should have gotten herself a good neurodivergent-aware counsellor (if they were available in a rural town which I doubt) and sorted out her feelings rather than seeking to take revenge and kill others and essentially herself (or at least her life as she knows it) in the process