1. Now Adam knew Eve his wife, and she conceived and bore Cain, and said, "I have acquired a man through God." 2. Then she bore again, this time his brother Abel. Now Abel was a shepherd of sheep, but Cain was a tiller of the ground. 3. Now in the process of time Cain brought a sacrifice to the Lord from the fruits of the ground. 4. Abel also brought a sacrifice from the firstborn of his flock and of their fat. The Lord respected Abel and his offering, 5. but He did not respect Cain and his sacrifices. So Cain was extremely sorrowful, and his countenance fell. 6. So the Lord said to Cain, "Why are you extremely sorrowful? And why has your countenance fallen? 7. Did you not sin, even though you brought it rightly, but did not divide it rightly? Be still; his recourse shall be to you; and you shall rule over him." 8. Now Cain talked with Abel his brother; and it came to pass, when they were in the field, that Cain rose up against Abel his brother and killed him.
And then, as we all know, Cain was kicked out of Eden, into Nod. Now, on to Exodus:
12. He who so strikes a man that he dies shall surely be put to death. 13. However, if he did not willfully lie in wait, but God delivered him into his hands, I will appoint for you a place where the slayer may flee. 14. But if a man acts with premeditation against his neighbour to kill him by treachery, you shall take him from My altar that he may die.
So. I think the issue here is that we don't have an exact timeline of event. The account obviously skips through time, since Eve has the boys, and the next thing you know they're full grown and making sacrifices to God. While we can ascertain that the death of Abel follows (likely closely) on the heels of God preferring one sacrifice over the other, it's not clear how closely.
If it had been premeditated murder, then God should have smote Cain, right then and there. But instead, He gave him a place to flee. Similar to the law He handed on through Moses.
And, on top of that, I still don't know just how much Cain would have understood about his actions. Yes, as Susanne pointed out, they clearly knew that animals could die, because Abel sacrificed one - but did they equate that with themselves? Adam and Eve knew that death was a consequence of their actions, but did they understand that that included death by violence or accident? Or merely old age? Did they even really understand what 'death' meant?