Allow me to explain how I view Mosaic law, at least in a simplistic way. 

I view the Mosaic law as done away with in its entirety, including the decalogue. I also believe things like murder and adultery are still sinful. How can this be? Is this a contradiction? No.

I view God's law as supra temporal, supra national, and supra cultural. That is, it's not bound by time, by nation, or by culture. 

Mosaic law, on the other hand, was given to a specific people for a specific time. It was never meant to be a binding legal document for all people of all time. As a matter of fact, MANY, if not most of the laws found in the Mosaic legal code were for the purpose of separation between the nation of Israel and the Gentile nations. Thus, it would be absurd to think it was ever, then or now, for Gentiles. 

—Ok. But what about things like murder and adultery? Was this for Israel alone? 

The Mosaic law code also included God's "moral law," as some call it, that would indeed apply to all people of all time. This is what I believe scripture calls "God's law" as found in 1 Corinthians 9 and is equated with the "law of Christ," also in 1 Corinthians 9. It is essentially the law to love God and love neighbor. Again, the law that applies to all people. 

Though the law of Moses contained elements regarding the love of God and neighbor, it does not spell out every practical way in which this law of love is to be applied. This is where the Spirit comes into play. The Spirit will guide us in how to apply this love law in any given circumstance. Put another way, we don't require a list, like that of Moses, because we have the Spirit to guide us. 

—But what about the unbelievers? They don't have the Spirit to guide them. How are they supposed to follow the law of love without a list? 

Israel was to be a light to the Gentiles. One of their missions was to show the world how to walk in relationship with God and neighbor. (They needed a list because they lacked the Spirit). We as Christians, as we walk by the Spirit, are to be the light of Christ to the world. In a sense, we become the "list" for the world. 

—Fine, but I'm still confused how you can say the decalogue has been done away with while simultaneously saying things like murder and adultery are sinful.

The decalogue was a binding legal agreement that had to be kept perfectly in order to stay in covenant. If you broke the agreement, you broke the covenant. The situation has changed in the new covenant. The "binding legal agreement" under the new covenant is faith, not obedience to any law. Therefore, the decalogue no longer has the LEGAL force it did under the old covenant. You cannot break the new covenant when you sin. As a legal binding document the decalogue has become obsolete. 

Christians don't need a long list of legal codes because we have the Spirit. Additionally, we don't need the specific legal codes of Mosaic law because we are not Israel. Not even believing Jews are bound by those legal demands. Those laws were for a literal nation during a very specific and limited time period. 

As Christians, we walk by the Spirit who guides us into loving God and neighbor which is called "God's law," or the "law of Christ." Some of these laws are found in the Mosaic code, but they're not longer "binding" in the same sense they were under the old covenant. All other laws found in Moses were specific to the nation of Israel and are no longer relevant under the new covenant and new Lawgiver. 

Murder and adultery are wrong today, as 2 examples, not because they're found in the decalogue, but because they're found outside the decalogue.