Contradiction with Matthew 5:9
Psalms 135:10 talks about God smiting kings, while Matthew 5:9 emphasizes peacemaking and being sons of God.
Matthew 5:9: Blessed [are] the peacemakers: for they shall be called the children of God.
Contradiction with Isaiah 2:4
While Psalms 135:10 mentions destruction of kings, Isaiah 2:4 speaks of nations not learning war anymore, emphasizing peace.
Isaiah 2:4: And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more. [pruninghooks: or, scythes]
Contradiction with Romans 12:19
Psalms 135:10 describes God taking vengeance, whereas Romans 12:19 advises against humans taking vengeance, leaving it 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.
Paradox #1
This verse involves God's actions in defeating other nations, which might conflict with the moral teaching of loving and forgiving enemies. Some might see a contradiction between divine justice and mercy, or between God commanding war in some parts and promoting peace in others.