Contradiction with Matthew 5:9
Judges 20:38 involves tactics for conflict, whereas Matthew 5:9 promotes peacemaking.
Matthew 5:9: Blessed [are] the peacemakers: for they shall be called the children of God.
Contradiction with Exodus 20:13
Judges 20:38 is part of a narrative involving battle and death, while Exodus 20:13 instructs not to kill.
Exodus 20:13: Thou shalt not kill.
Contradiction with Romans 12:19
Judges 20:38 describes a strategy for vengeance in battle, whereas Romans 12:19 advises to leave vengeance to God.
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 25:21-22
Judges 20:38 is about warfare strategy, while Proverbs 25:21-22 advises to feed and give drink to your enemy.
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 Luke 6:27
Judges 20:38 deals with war, while Luke 6:27 advocates loving enemies.
Luke 6:27: But I say unto you which hear, Love your enemies, do good to them which hate you,