RANCE ADAMS
Member
The OT definitely has application to Christians and is vital for Christianity and is an explanation for the New Covenant and Jesus. It just is not the guide from God on how to live and spend eternity in God's presence. He sent Jesus as His final prophet/messenger to give us His direction/desire for us.There is a lot more scripture in the OT that we can study to determine what God's moral principles are, besides the Law of Moses (aka. old covenant) that you refer to. To be sure those same principles will be found before the cross and after the cross.
So when Cain killed Abel we can determine that killing, in that case, looked a certain way and God said it was wrong and why it was wrong. God's moral principles or law.
In your last sentence I'm not sure who you are referring to as being 'us' and 'we'. But that is a different topic not included in this discussion.