Murder is the unlawful taking of a human life, where killing is the lawful taking of life. All murder is killing but not all killing is murder. Protection from unlawful killing is taught in Exodus 20:13 and Deut. 5:17.
Since all have sinned against God, He has the right to execute (kill) them because they are lawbreakers worthy of death. God cannot murder an individual because that would entail an unlawful taking of life, which God cannot do.