Definition of murder: kill intentionally and with premeditation; "The mafia boss ordered his enemies murdered"
Taking someones life with the intention of doing so IS murder. No matter which way you look at it. Just because the armed forces are fighting to protect our country does not mean they aren't murderers if they kill some one. What would you call it? Generous killing? No, you takes someones life then you are a murderer by definition. End of.
Some examples would be acceptable though. Euthanasia for one, if my father has been withering away with disease and is terminal and asks that I assist him to go with dignity... that would fit the definition you have posted there. I would be killing him intentionally and it would be premeditated, but not sure I would feel that I was a murderer for doing it