He does not say it was his belief; he is calculating that those people, invested in the slave institutions, would believe in the evil (a word normally used in those times to mean 'mistake', 'wrong', etc.), of non-slave states actions to undermine the slave institutions. His main idea ending in these words...

"Should so dangerous a mode of believing once take root, no one can calculate the consequences." The words "a mode of believing" sounds like he doesn't believe it the "evil" himself, but others would.

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David Upton