Contradiction with Exodus 20:13
This verse commands "Thou shalt not kill," which contradicts any context of war or battle implied in 1 Chronicles 12:13.
Exodus 20:13: Thou shalt not kill.
Contradiction with Matthew 5:44
Instructs to "Love your enemies," which goes against the notion of gathering for battles or conflicts as might be derived from 1 Chronicles 12:13.
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 Isaiah 2:4
Describes turning swords into plowshares, which contradicts the militaristic gathering context in 1 Chronicles 12:13.
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]