Living Blood

Once Kayin had killed Hevel, the voice of his brother's blood called to G-d from the ground.
G-d (mercifully) questioned Kayin concerning the disappearance of his brother. Kayin's response said it all. First, he lied, stating that he didn't know where Hevel was. Then he answered G-d with complete disrespect, asking sarcastically if Hevel's whereabouts were his responsibilty. The irony here is incredible - and directly to G-d's face! On top of that, he showed absolutely no remorse.

However, all of that is secondary to my questions.
How did Kayin know it was wrong to murder?
How was G-d just in punishing Kayin if he had not previously instructed him not to kill?
Why did G-d choose to put a mark on Kayin, warning of vengeance if any should take his life, rather than just allowing whatever might befall him?
Was Kayin preserved as an example?
If so, what was he an example of?
Could he possibly have had an agreement with G-d of which we're unaware?

Living Word

How did Kayin (Cain) and Hevel (Abel) know to offer sacrifices to G-d?
Hevel offered the firstborn of his flock. Why?
Were these sacrifices offered for sin?

In Genesis 4:6, G-d tells Kayin that he will be "accepted if he does well", but that "sin crouches at the door" if he doesn't "do well". This is a strange warning from G-d considering He hadn't yet given His instruction, isn't it? How would Kayin even know what sin was at that point in time?