As Wesley J. Smith points out, when he was in law school he was taught that X's agreeing to be murdered does not make killing X legally something other than murder. And a good thing, too. Well, as usual, the UK is leading us into a new world. In Brave New Britain, that isn't true anymore, evidently.
Michael Bateman put a bag over the head of his wife Margaret and pumped in gas to kill her. She was in a lot of pain that wasn't getting properly treated because the moronic medical establishment didn't diagnose her broken pelvis. (It was discovered after her death.) She was also depressed about not being able to do normal things like taking showers. That's it. She wasn't dying, if you think that's a relevant consideration. (I don't.) So she and Michael planned her death, and he killed her, because she apparently couldn't do it herself. The prosecutor has declared that it "isn't in the public interest" to prosecute Michael, presumably because Margaret agreed to be murdered.
In the current context of recent posts, I'm almost afraid to ask whether mere libertarianism would mean that agreeing to have yourself killed with plastic bags and gas means that everything is A-okay and no prosecution should be carried out against your killer. I'm pretty sure I know the answer. And that, folks, is just one reason that, while I'll defend small government all over the place, I don't carry a libertarian card.