Abel had faith. Cain didn't. Evident by their offerings... Cain brought crops (his own works of self) whereas Abel brought a blood sacrifice as God had instructed. Cain didn't believe God and did his own thing. This same thing can be seen today... those who have faith in the blood of Jesus as atonement for their sins versus those with no faith, who depend on their own works for salvation!!
Jude speaks of apostates who don't have the Spirit (Jude 19) and likens them to Cain!
Jude 11...
"But I want to remind you, though you once knew this, that the Lord, having saved the people out of the land of Egypt, afterward destroyed those who did not believe. And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day; as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the vengeance of eternal fire.
Likewise also these dreamers defile the flesh, reject authority, and speak evil of dignitaries. Yet Michael the archangel, in contending with the devil, when he disputed about the body of Moses, dared not bring against him a reviling accusation, but said, “The Lord rebuke you!” But these speak evil of whatever they do not know; and whatever they know naturally, like brute beasts, in these things they corrupt themselves. Woe to them! For they have gone in the way of Cain, have run greedily in the error of Balaam for profit, and perished in the rebellion of Korah."
Hebrews 11:4: "By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks"
1 John 3:12: "...not as Cain, who was of the evil one and slew his brother. And for what reason did he slay him? Because his deeds were evil, and his brother’s were righteous"
Matthew 7:23: (judgment day) ".....depart from me, you workers of lawlessness"
John 8:44: "You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it"