I have a basic understanding of implicature. Saying that you are permitted to kill patients if you are doing it incidentally in the process of alleviating pain also implies, in normal conversation, that you are not allowed to do it otherwise. It is a "limiting implicature".
Outlining how in some jurisdictions you're allowed to drive drunk in some situations and giving the example of escaping a kidnapper may imply that driving drunk is normally disallowed. Absolutely.
On the other had it in no way shape or form implies that similar rules of thumb do not apply if you're escaping a rapist or murderer.
Talking about a specific case where it's ethical to relieve a particular kind of distress through death implies that using death as a treatment isn't the usual case but it in no way shape or form implies that it's the only case.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.