What possible evidence do you have to suggest that “most” of his fans are “actually disgusting racists”?

These people are obviously idiots, but I still don’t generally understand why slippery slope is a rhetorical fallacy. Doesn’t our court system work on the basis of precedent? If X is redefined for the sake of reason Y, doesn’t that mean it will be easier next time to redefine X for the sake of reason Z?