Here we go...
Again.
What some fail to understand is that the definition of Murder has to do with the "Unlawful" intentional killing of another (excluding exceptional circumstances of self defense) with regard to the general population.
The punishment for committing such crime is "Lawfully" carried out by the authorities as the penalty for committing said crime. 
Definition:
murder   Show phonetics
 noun [C or U] 
the crime of intentionally killing a person:
Two sisters have been 
charged with (= officially accused of) murder.
There were three murders in the town last year.
The three were 
convicted of (= proved guilty of) murder.
murder 
weapon (= a weapon used to commit a murder)
Compare 
manslaughter; 
suicide (DEATH).
murder   Show phonetics
 verb [T] 
1 to commit the crime of intentionally killing a person:
Her husband was murdered by gunmen as she watched.
In the last year, terrorists have murdered several local journalists.
2 UK INFORMAL If you say you will or could murder someone, it means you are very angry with them:
If he's late again, I'll murder him!
murderer   Show phonetics
 noun [C] (OLD-FASHIONED FEMALE murderess) 
someone who illegally and intentionally kills another person:
A 
convicted murderer was executed in North Carolina yesterday.
a 
mass murderer (= someone who has killed a large number of people illegally)
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penalty   Show phonetics
 noun [C]
1 a punishment, or the usual punishment, for doing something that is against a law
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the death penalty noun 
the legal punishment of death for a crime.
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law (RULE)   Show phonetics
 noun [C or U] 
a rule, usually made by a government, that is used to order the way in which a society behaves, or the whole system of such rules:
There are laws against drinking in the street.
The laws governing the possession of firearms are being reviewed.
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the law noun 
the system of rules of a particular country:
Of course robbery is against the law!
You can't take that course of action and remain within the law.