(er / son, child, person)

Genesis 4:1-2
Adam lay with his wife Eve, and she became pregnant and gave birth to Cain. She said, "With the help of the Lord I have brought forth a man." Later she gave birth to his brother Abel. NIV
Then Adam had intercourse with his wife, and she became pregnant. She bore a son and said, "By the Lord's help I have gotten a son." So she named him Cain. Later she gave birth to another son, Abel. TEV

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(er / son) + (kou / mouth) = (xiong / elder brother)

The first born son is considered to be the spokesman (mouth) for the family.

Cain and Abel offered sacrifices to God out of their agricultural production. However, Cain in defiance to God's direction, offered something different and was criticized for it. In his stubbornness Cain killed Abel.

Genesis 4:8
Now Cain said to his brother Abel, "Let's go out to the field." And while they were in the field, Cain attacked his brother Abel and killed him. NIV
Then Cain said to his brother Abel, "Let's go out in the fields." When they were out in the fields, Cain turned on his brother and killed him. TEV

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After a confrontation with God about his evil deed, Cain was banished from the region. He was given a mark to identify him as a bad person.

Genesis 4:15>
But the Lord said to him, "Not so; if anyone kills Cain, he will suffer vengeance seven times over." Then the Lord put a mark on Cain so that no one who found him would kill him. NIV
But the Lord answered, "No. If anyone kills you, seven lives will be taken in revenge." So the Lord put a mark on Cain to warn anyone who met him not to kill him. TEV

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(xiong / cruel, violent, fierce, terrible, evil, bad)
The Oracle Bone figure clearly shows the marked elder brother grabbing his brother to kill him. Some of that detail is lost in the traditional version, but the mark is still present.