Contradiction with Exodus 20:13
Psalms 136:17 mentions striking down great kings, whereas Exodus 20:13 commands, "Thou shalt not kill," which contradicts the act of taking life.
Exodus 20:13: Thou shalt not kill.
Contradiction with Matthew 5:44
Psalms 136:17 celebrates the defeat of kings, while Matthew 5:44 instructs to love your enemies, not harm them.
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
Psalms 136:17 speaks of defeating kings, but Romans 12:19 advises against seeking vengeance, stating vengeance belongs 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.