Not all killing is classed as unlawful. A policeman shooting someone armed with a gun in self defense could be deemed as lawful. A gunman shooting dead a policeman would definately be classed as unlawful.
Lawful killing includes self defence, a death occurring in the prevention of crime or during an arrest, misadventure or accident, killing the enemy in time of war and a lawful execution. Our country does not now execute people, but there could be cases where the death occurred elsewhere. This answer is very much an over-simplification of the law and each case would be decided on the facts.
As one answer said, self defence can be termed as this, but generally it is where a death has occured where no intent was present, i.e it was more of an accident than intended.
naomi24 This should answer your question, hopefully. Voluntary manslaughter occurs when all the elements of murder are present, including an intent to kill or cause grevious bodily harm, but the crime is reduced to manslaughter by reason of provocation, diminished responsibility or death (suicide pact). Involuntary manslaughter is unlawful killing without the intent to kill. Apart from the intent element all other elements of the offence are the same as in murder.