Contradiction with Matthew 5:39
This verse teaches to resist not evil and turn the other cheek, contradicting 2 Samuel 4:9 where David commends the slaying of his enemy.
Matthew 5:39: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
Contradiction with Romans 12:19
This verse advises against seeking vengeance and leaving it to God, contrasting with David's reaction to his enemy's death in 2 Samuel 4:9.
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 Proverbs 24:17
This verse warns against rejoicing at the downfall of an enemy, which can contradict the tone of 2 Samuel 4:9 where David discusses the defeat of his foe.
Proverbs 24:17: Rejoice not when thine enemy falleth, and let not thine heart be glad when he stumbleth:
Contradiction with Exodus 20:13
The commandment "Thou shalt not kill" can be seen as contradicting David's acceptance of the act of killing in 2 Samuel 4:9.
Exodus 20:13: Thou shalt not kill.
Contradiction with Proverbs 25:21-22
Advises showing kindness to enemies, which contrasts with the violent reprisal found acceptable in 2 Samuel 4:9.
Proverbs 25:21-22: If thine enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink: