Contradiction with 2 Samuel 3:27
This verse describes a different murder setting, while 2 Kings 12:21 details a separate assassination, contradicting the motivation and context of the incidents.
2 Samuel 3:27: And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth [rib], that he died, for the blood of Asahel his brother. [quietly: or, peaceably]
Contradiction with 1 Samuel 24:6
Here, David refuses to harm Saul because he is the Lord's anointed, which contrasts with 2 Kings 12:21 where the Lord's anointed king is assassinated.
1 Samuel 24:6: And he said unto his men, The LORD forbid that I should do this thing unto my master, the LORD'S anointed, to stretch forth mine hand against him, seeing he [is] the anointed of the LORD.
Contradiction with Proverbs 20:22
Advises against seeking personal vengeance, contradicting the act of murder committed in 2 Kings 12:21.
Proverbs 20:22: Say not thou, I will recompense evil; [but] wait on the LORD, and he shall save thee.
Contradiction with Exodus 20:13
The commandment "Thou shalt not kill" directly contradicts the murder described in 2 Kings 12:21.
Exodus 20:13: Thou shalt not kill.