Why did many Southerners believe that they were the victims of an antislavery conspiracy?

I think that first you have to realize that many people tend to
feel that there is a conspiracy against them when things don't go their way.  That is why, for
example, the Tea Party thinks Obama is trying to make America become socialist and/or Muslim and
why liberals thought that Bush was trying to turn the US into a theocracy or a country run by big
business.  Therefore, it is not that surprising that Southerners thought there was a conspiracy
when they did not get their way.

This sort of thinking was spurred
on whenever events seemed to go against them.  For example, when the Wilmot Proviso was inserted
into various bills around the time of the Mexican-American War, they thought that showed the
government was against them.  Or when Northerners resisted the Fugitive Slave Act, they felt the
same.  Finally, when an anti-slavery candidate (Lincoln) was elected, they felt that proved that
the North was conspiring to end slavery forcibly.