Contradiction with Matthew 5:44
Matthew 5:44 instructs to love your enemies and bless those who persecute you, contradicting the act of slaying an enemy as described in 2 Samuel 23:21.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Contradiction with Romans 12:19
Romans 12:19 advises against taking revenge, whereas 2 Samuel 23:21 describes an act of taking revenge by killing an adversary.
Romans 12:19: Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord.
Contradiction with Exodus 20:13
Exodus 20:13 commands "Thou shalt not kill," which contradicts the action of striking down an enemy in 2 Samuel 23:21.
Exodus 20:13: Thou shalt not kill.
Contradiction with Proverbs 25:21-22
Proverbs 25:21-22 encourages feeding and giving drink to your enemy, in contrast with the slaying of an enemy in 2 Samuel 23:21.
Proverbs 25:21-22: If thine enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink:
Contradiction with Matthew 26:52
Matthew 26:52 states that "all they that take the sword shall perish with the sword," conflicting with the commendation of a warrior's deed in 2 Samuel 23:21.
Matthew 26:52: Thinkest thou that I cannot now pray to my Father, and he shall presently give me more than twelve legions of angels?
Paradox #1
The potential contradiction in this verse could be the idea of using violence or killing as a means to resolve conflicts or assert dominance. This might conflict with other teachings in the Bible that emphasize peace, forgiveness, and loving one's enemies.