It wasn’t much of a murder mystery: there was Cain arguing his innocence, Abel missing, blood in the paddock crying out against the murder, and God asking the survivor where his brother was.
It was the first recorded murder.
It started with sin: the sin of Cain offering any part of his crop up to God rather than the best:
Genesis 4:3–4 When it was time for the harvest, Cain presented some of his crops as a gift to the Lord. Abel also brought a gift—the best portions of the firstborn lambs from his flock.
The sin of envy and pride:
Genesis 4:4–5 The Lord accepted Abel and his gift, but he did not accept Cain and his gift. This made Cain very angry, and he looked dejected.
The sin of winning by any means. In this case, by eliminating the competition for God’s favour: killing Abel.
Cain could have avoided all this by taking God’s advice:
Genesis 4:7 You will be accepted if you do what is right. But if you refuse to do what is right, then watch out! Sin is crouching at the door, eager to control you. But you must subdue it and be its master.
What was right would have been repentance, for example, as King David described in Psalm 51, in which David repents, begs forgiveness of his sin, and begs God not to banish him from His presence (vs 11).
Cain, instead, accepted his fate, sort of. After God assured him of his physical safety, Cain left God’s presence and settled elsewhere. Apart from listing a few generations of descendants, the Bible records nothing of Cain again.