The evil, as Calhoun defined in your quote, could be one of two things. Either disunion itself, or "a belief" which would bring about disunion.

"A belief in the slaveholding States that it is the intention of the other states gradually to undermine their property in their slaves."

Evil could be defined as something which should not be, and good men should act against.

IMHO Southern beliefs were well founded, but blame for bringing the crisis of disunion about can be directed North and South.