> It's wonderful that they may have helped you at one point.
That's not what they said, you're misreading.
> Can we please see some formal, peer-reviewed evidence to suggest that someone willing to commit murder is going to be deterred by a DUI-equivalent?
I did a very quick search using neutral terms for studies on this. Here are the first three I found.
> PO victims had police incident rates that were more than double the matched group prior to the PO, but dropped to the level of the matched group during and after the order. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4151113/
> Results showed significant reductions in threats of assault, physical assault, stalking, and worksite harassment over time among all women, regardless of receipt or nonreceipt of a protection order. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448307/
> Permanent, but not temporary, protection orders are associated with a significant decrease in risk of police-reported violence against women by their male intimate partners. https://jamanetwork.com/journals/jama/fullarticle/195163
That last one had a pretty good study size - 2691 women.
I don't think it's incredible that a protective order could stop an expanding pattern of abuse and violence before it becomes severe enough for a murder to occur. You say "someone willing to commit murder", and it's probably true that if the culprit is already very willing or actively intending to commit a murder, a protective order is useless. Abusers cannot be placed into permanent "murderous" and "non-murderous" groups, though.