While it's almost certain they all had contracts out on them, Astrid does prefer you killing one particular person over the others:
Vasha, the Khajit, describes himself as a murderer, a rapist, and a thief. If you kill him, Astrid remarks that he was the obvious choice.
However, if you kill all of them, she'll make a snarky comment that you're an overachiever.
But as you'll soon find out—should you choose to join the Dark Brotherhood—the contracts placed are, many times, non-obvious and counterintuitive.
Basically, the point of the abandoned shack test is to get you to kill on command: if the Family says someone needs to die, that person needs to die, regardless of what excuse they give you or backstory they have.