“Properly that commandment should be read thou shall not murder.”
That is really splitting hairs you know. Cain killed Able, but if he were tried in a court of law today more than likely he would be found guilty of manslaughter.
Then there is 1st and 2nd degree murder.
Killing is killing, someone is dead by your hands does not matter how it happened.
Murder is essentially the unjustified taking of a life. All killing is not murder. If person A is in the process of trying to kill you and you manage to kill them instead it’s not murder. Killing an enemy during a just war in generally isn’t murder though it can be (under our law, Geneva Convention and Catholic Just War Doctrine.). For example killing an unarmed soldier who has surrendered and doesn’t present a threat would in most cases be murder.
We have chosen to split murder into a number of different categories and given them different names to reflect their threat to society. Conscious premeditated unjustified killing being the most serious down to the accidental killing of someone being the least serious. Even premeditated murder has levels depending on threat to society. We usually refer to those as enhanced circumstances such as killing a cop or judge.