Can a secular government ever be a moral authority? Can a secular government ever be a moral authority?  For example, if there comes a day in the future that a secular government decrees that murder is no longer a criminal offense in that nation, should a Christian living in that nation feel confident that they do no wrong if/when they murder someone because they strongly believe that their secular government is a moral authority?

Furthermore, when that Christian dies and stands before God on Judgement Day and they tell God that they strongly believed that all the laws passed by the secular government were morally good, will God automatically forgive that person of the murder(s) they committed?
Normally we require questions like this to specify a particular denomination/tradition that they are asking about, because different Christian groups take different approaches. But in this case Christian groups take a remarkably uniform position.

The government is not and cannot be a higher moral authority than God. No government can declare something to be good that God has decreed to be bad, nor something bad that God has decreed to be good. 

However one of the moral goods that God has commanded is to submit to secular authorities - within limits. For example, if the government says that you should not drive faster than 70mph, or consume certain drugs, then Christians should obey those while under the authority of that government. The existence of such a law does not make driving over 70mph bad per se, but nonetheless Christians should refrain from doing it.

Governments are not permitted to declare, for example, that worshipping God is illegal, or that someone else other than God must be worshipped. This principle is demonstrated many times throughout the Bible, in both Old and New Testaments, and Christians are expected to ignore those rules.

Governments are also not able to declare anything 'permitted' that is not permitted by God. So your example of a government that decreed murder to be legal would not make it so, and Christians could not for that reason commit murder. This is demonstrated by the governments that have - to an extent - done just that, by permitting abortion or euthanasia. Christian groups that consider those to be murder (of which there are many) have not at all permitted their members to carry out those acts just because the government says it is OK.

There are a small number of Christian groups that consider secular governments to have no authority at all over Christians. They would disagree with the above, but even they would generally expect their members to follow secular laws as a matter of practicality.