Sinfulness is partially defined as the ways people seek to cope with life that are inherently self destructive like substance abuse, gambling addictions, etc. You do not require any faith in God to believe these sinful behaviors lead to both physical and spiritual death, you only need to recognize behavior which violates life… your own or others. You seem to struggle more with the word “sin”, than the consequences of “sinful” behavior. I suspect that has to do with your rejection of “God”.

Various social scientists believe there is a “God” gene, so without God people become lost and unanchored and that leads to destructive behavior.

Karl Marx wrote about the need of people to believe in “God” and that belief could be used by redirecting it towards “faith in the state”. Freud said people need “parents” and when they are to old for their biological parents, they seek parents in either the concept of God, a heavenly and spiritual father. Anthropologists have identified this “God tendency” by noting virtually every society ever known to man has worshipped some deity. Secular psychologists have said God is a coping mechanism man needs because he is both “self aware” and able to understand their mortality.

I believe God created us to have a relationship with Him. And when we opt not to we must “cope” with that missing piece of our lives in other ways. Many of those ways turn out to be destructive or compulsive behaviors. Regardless it ends the same way… destructive or compulsive behaviors.