Cain was "of the Devil" - 1 John 3:12 (a hint at the future)

[beameup]

Not as Cain, who was of that wicked one

For this is the message that ye heard from the beginning, that we should love one another. Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous. - 1 John 3:11-12

The eternal destiny of believers will be one of love.
And vice versa

 

[beameup]

By faith Abel offered to God a better sacrifice than Cain, through which he obtained the testimony that he was righteous, God testifying about his gifts, and through faith - re: Hebrews 11:4
Able, by FAITH, offered up a BLOOD SACRIFICE to God.
This hints that Able recognized the magnitude of his sinfulness.
though he [Abel] is dead, he still speaks [by example].

 

[beameup]

Woe unto them! for they have gone in the way of Cain - Jude 1:11a

For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened. - Romans 1:21

Obviously Cain "knew God" but departed from God and followed Lucifer-Satan-The Devil.