I watched Pat Brown criminal profiler's youtube video yesterday on the topic of motive in the case of confessed mass murderer Bryan Kohberger (Idaho 4 murders). He wasn't asked to explain why he did it when he took a plea deal to remove the death penalty, and she said it wouldn't have been useful to ask him (or any mass murderer or serial killer) in any case because they are in the main (always IMO) psychopaths, pathological liars and only tell the truth when it's useful to them. She went on to say that there is always only one motive in these types of murders and that is power and control.
IMO that also applies to Erin. Whether it was a situation in which she bore a grudge or wanted to punish (some or all of) them, or exact revenge for the way she was treated or not respected, or didn't get what she wanted from them or they didn't love her, etc, she had no power or control over them, and that is the ultimate way in which she would exert power and control.
She would have no remorse to plan such atrocious murders, and a remorseless individual would not care whether innocent bystanders such as the Wilkinsons were also sacrificed with the aim of carrying off her plan of coercing Simon to attend. IMO