To aqwsed:
While my question was a genuine one (one that I have meditated upon for many years) my fear that you would legalistically answer has it, has unfortunately proved true. Much like when defending the Trinity through a voluminous text, one only finds the last sentence to ultimately read that "it is a holy mystery", at the moment your answer have proved to be unsatisfying vague.
No, the problem I have is simply ... WHY.
Why did our loving heavenly Father not say to Able "it is a good idea to not go out with your brother today"?
I ask this question because even my actual fleshly and sinful father warned me many times, not to climb on the roof, light fires in the garden or play on the road because I MIGHT have a accident ... but yet God did not warn righteous Able, especially when he a very, very good idea, what was going to happen next.
I could understand God's inaction if he felt that he'd done enough for Cain ... but no. As we learn latter in the Genesis account, Jehovah talked to Cain again and even arranged for his physical protection.
Thus it hardly seems fair that Abel led a life of self-sacrificing service, when he didn't get the same consideration as a sinner.