First of all, God isn't a murder. Why not? God owns everything. So, even if He takes life away, He was the One Who gave it in the first place. It is His by right, to do with as He pleases.
Murder is purposely killing a person. Whether or not you own that person is not in the definition - the only important thing is whether or not the murder was "lawful".
If God exists and is omnipotent, or even only omniscient, it will obey no human laws and so if this God kills a person in a way that is not allowed by the laws of a particular nation, it does so unlawfully. It doesn't matter that you think this God's law should have primacy.
Given the above (God exists, God is at least omniscient) then if a person's death results from this God's actions in a way considered unlawful by a given nation, then that God has committed a murder in that nation.
Interestingly this means that God cannot commit murder in countries that have religious laws since I'm fairly certain they would by definition exempt their God from any crime.