There is not enough information to make that speculation, but it would be something to do with rage (as in, the fruit of wrath - "the same spirit that operates in the sons of disobedience").
God warned Cain in the provided passage I gave you:
Genesis 4: ESV
3In the course of time Cain brought to the LORDan offering of the fruit of the ground,
4and Abel also brought of the firstborn of his flock and of their fat portions. And the LORD had regard for Abel and his offering,
5but for Cain and his offering he had no regard. So Cain was very angry, and his face fell.
6The LORD said to Cain, “Why are you angry, and why has your face fallen?
7If you do well, will you not be accepted?
b And if you do not do well, sin is crouching at the door. Its desire is contrary to
c you, but you must rule over it.” (ESV)
Then he killed his brother.
Romans 3: ESV
“None is righteous, no, not one;
11no one understands;
no one seeks for God.
12All have turned aside; together they have become worthless;
no one does good,
not even one.”
13“Their throat is an open grave;
they use their tongues to deceive.”
“The venom of asps is under their lips.”
14“Their mouth is full of curses and bitterness.”
15“Their feet are swift to shed blood;
16in their paths are ruin and misery,
17and the way of peace they have not known.”
18“There is no fear of God before their eyes.”
Cain, having never known what murder is, in the moment of rage picked up a rock without giving it a second thought.
Which sounds like what a decent defense attorney would evoke for a plea bargain.
Cain had no such knowledge of how to handle the rage and murder came about because the sin got the master of him.
He sure did. God advised him in the Genesis quote I posted now for the second time.