As to motive ... you only touched lightly, let's hear your thoughts. Creighton affiliation? Also was Shirlee or Tom the target? BBL.
My general leaning, based on the thoughts above, is Shirlee as the target. Possible scenario:
Single perp, likely representing others, comes looking for Shirlee at her place of work in order to 'discuss' something serious (very possibly linked to the various oddness/rumours involving her daughter's possible bad debts/recent burglary at Shirlee's place/Shirlee carrying large amounts of cash around with her, etc - did we look further into any of those?). At this point, he's armed but not
necessarily with full intent. Doesn't get the right answers, and things escalate.
I can think of numerous reasons why it would be advantageous to make this sort of approach at the Hunters' house, at this hour, as opposed to the more obvious route of approaching Shirlee at her own home - more anonymous, more isolated, fewer options for her, maybe she's stopped answering the door to strangers at home, etc. Plus in the event of a crime, the venue certainly complicates the investigation, as this thread shows...
It also explains why she might have let him in, which seems odd in itself, as well as the fact that the POI didn't look/act like a random drifter and that he only had a rough knowledge of the address.
Saddens me immensely to say so, but I'm inclined to think that Tom might possibly have been a really, really tragic collateral victim when things didn't work out satisfactorily for the visitor.
Obviously there are many things I think disagree with this theory - not least that taking out a kid, even as a potential witness, is a hell of a jump - but this seems to fit better than anything else I can think of for now. Could, of course, be way off - all JMO, and pure guesswork/gut feeling.