Random crimes like this are rare, but I’ll humor you.
I’ll use the Cheshire murders as a bit of a model, although that was a robbery that turned into rape and multiple murder.
I’m a drug using criminal with a history of sexual deviance. I have a gun in my vehicle.
I encounter Jayme and her mother at some point during the day of the attack. I want Jayme.
I follow them home, and discover to my delight, that the house is relatively secluded.
I leave, get something to eat, and perhaps use drugs or alcohol.
I engage the house late that night, when I know the family will be sleeping. I kill the parents, and kidnap Jayme.
I escape, not by good planning, but with good luck.
Now I don’t think this is what happened, but that’s how a random crime could occur.