We agree on that. But the rest of your post really cuts against the argument that these guys were unable to make a choice.
By pointing to Ted Bundy, you really make my point. Bundy was smart, together, personable and clearly in control of his life and able to make choices and to understand them. It's just that the choices he made were very very evil.
The victim here looks somewhat like a masochist's version of Ted Bundy. Employed, responds to an ad appropriately (ie he got what the ad promised) and participated voluntarily in the slaughter and the rest of that sick stuff voluntarily. My take on this is that the guy was able to make and understand his choices, its just that, like Bundy, his choices were also evil.
We give Bundy a trial and an execution precisely because he understands his choices. Of course, Bundy's choices did not involve choice by his victims.
That's what makes this article interesting. There was clearly no coercion and the victim seems perfectly capable of making choices and understanding them.
It seems to me that Libertarians ought to be giving the victim here the right to make his own choices, no matter how weird, as long as they don't hurt someone else.
After all, the perp here looks like he is just helping the victim in an assisted suicide--a really weird one, but an assisted suicide nevertheless. In that case, why should Libertarians put the perp in jail?