When I read Isaiah 1, I can't help but hear echoes of the story of Cain. I'm sure this has everything to do with the fact that I have been thinking and posting about Genesis 3-4 lately. Hear me out, and let me know what you think. First the text, and then an explanation.
Doing and getting "good" (טוב, red text): The context of Isaiah 1:18ff. is similar to that of Genesis 4:6ff. In both passages, God addresses the guilty party. In both passages, the terms of law are stated similarly--the combination of the אִם conditional clause and the טוב root.
Spilling blood (דם, yellow text): In Isaiah 1, the hands of the people of God are said to be "full of blood." Cain washed the blood off his hands, yet it cried out to the Lord from the ground.
Murder (הרג ,רצח, green text): This makes explicit that the bloody hands of Isaiah 1 are those of a Cain-like, murderous people.
How/Where (אי ,איכה, purple text): The connection here is more clear in Hebrew, but still, some might think it a stretch. To me, this expression resonates with Genesis 4 more loudly than any of the others, besides the טוב connection. In a previous post, I mentioned the similarity between Genesis 3:9 and 4:9. After Adam sinned, the Lord asks, “where are you (איכה)?" After Cain killed his brother, the Lord asks, “Where is Abel (אי הבל)?" So when I hear אי in Genesis 4, I think of איכה in Genesis 3. Though איכה is usually translated "how" in Isaiah 1:21, in light of the previously mentioned connections, when I read איכה in Isaiah 1, I recall the violence of Genesis 4.
What does all this matter? Hearing the echoes of Cain in Isaiah 1 adds another layer of richness and meaning to both Isaiah 1 and the story of Cain in Genesis 4. Violence, no less than that of Cain, crouches at the door. You don't have to slash someone with a knife to be in Cain's predicament. Turning a blind eye to "justice and righteousness" will suffice (Isaiah 1:21-23).