During war, it's still homicide. Murder is a crime, always.
Self defense isn't murder, it's homicide.
Hit men/CIA ops/whatever are government agents acting under the confines of the law that justify their homicides. Therefore, no murder.
Homicide is only murder if it is an unlawful killing. Self defense, war, and assassinations are lawful (assassinations you might be able to take issues with, but that's a hugely complex things of international law and treaty)