A mundane reason for snipped power cords & detonated robots? If we don't kill all the humans in this first attack, they'll get suspicious/antsy, and start to suspect
things. Not good for a second attack, nor for staying hidden.
If we do pull off everything perfectly, how do we intend to keep the ship functional? We still don't know enough about their tech, or language, and would have to ferry our science drones & lab equipment out there to do so.
If we don't keep the ship functional, it starts appearing on the military's radar. Hmm, that's strange, this pirate vessel's turned into a ghostship. Signs of struggle & sabotage....just a handful of km from that (suspected) xeno ship.
I suspect they're quite a ways- it took what, 2 days for our stimmed-up fliers to get there?
@Morrigi
Nonono! Just because we're already here doesn't mean we have to stay here.
Sunk cost fallacy.
-edit
What do you fellows suggest we do once we've wiped out the crew? How do we handle the ship itself? What about the affiliates of the men on-board?
As an example, we might be bringing down the personal interest of some badass space-bounty-hunter type, ("My brother was on that ship, and I'm going to find him. I don't know who you are. I don't know what you want. If you are looking for ransom, I can tell you I don't have money. But what I do have are a very particular set of skills; skills I have acquired over a very long career. Skills that make me a nightmare for people like you. If you let my daughter go now, that'll be the end of it. I will not look for you, I will not pursue you. But if you don't, I will look for you, I will find you, and I will kill you.")