The man who is alleged to have killed the these people (two adults and four children) is also said to have a history of domestic violence and had a "protective order against him."

Utah police said Melannie Haskell [ex-wife of the alleged killer] in August 2013 reported a protective order violation in which she alleged Ron threatened her and her lawyer. She alleged another such violation that October, saying that her then-estranged husband had shown up at one of their children's elementary schools.
As police noted, "neither protective order violation was prosecuted."
Whether the man is responsible for the murders or not, what is certainly true about this society, and what anyone who has read police reports in this and other cities knows for certain, is that the person a woman should most fear is not a terrorist or a stranger in the shadows of an empty street but the man they live with, eat with, share a bed with. Which is why a woman buying a home security system is really nothing but money wasted. From Futures Without Violence (a PDF)
94% of female murder victims killed by men are killed by a man they knew. In other words, females are 16 times as likely to be killed by a male acquaintance than by a male stranger. In 2010, 1,017 women, almost three a day, were killed by their intimate partners.
Clearly, the Domestic Violence Offender Gun Ban, which was enacted in 1996, is not strong enough the make a real social impact and protect the lives of American women.
Court records show that in Utah in 2008, Haskell was charged with domestic violence and simple assault against his wife. She reported that he had hit her in the head and dragged her by the hair, according to police and court records. He pleaded guilty to the assault charge and had the domestic-violence charge dismissed as part of his plea deal. In July 2013, Haskell's wife filed a protective order against him in Cache County, Utah, where they lived at the time. The order applied to her and their four children. She then moved away and filed for divorce about a month later. The divorce was finalized this past February.

It's not yet clear if Haskell possessed his guns legally, but his case appears to be the latest example of how easy it remains for domestic abusers to possess firearms, thanks to weak legislation.