who is the real murderer?
Since this happened after the Rebellion God is not oblidged to intervien. If the Rebellion hadn't occured and Cain raised his hand to murder Able yes, God would have protected Able. The issue of God's Law vs Lawlessness was now in play so He was not and is not oblidged to act. After Armageddon lawlessness will not be tolorated.
So 'who is the real murderer?' Cain is, he ignored God's personal warning to him and used his free will for evil.