Which is how in the US death penalty is executed. Even the pharmaceuticals use are, as far as I know, either the same or very similar: http://en.wikipedia.org/wiki/Sodium_thiopental

Of course, while this is a very humane way to execute an euthanasia (either of animal or a human), talking about the least inhumane way to carry out an execution is somewhat ridiculous. The most humane way is to just not do it.

Also, it's totally off-topic.