Why were loyalists to the British crown held in such low regard and treated with abuse iby the Patriots in the American Revolution?

The reason
for this is that they were people who might be expected to be on the side of the Patriots yet
who were not.  No one would have expected a British soldier to be on the Patriots' side so they
would not have been hated.  But Patriots might well have expected that their own neighbors would
be on their side.  Therefore, when they were not it would have seemed like something of a
betrayal.  This would have led to the Tories being hated and abused more than might seem
natural.

Because the Loyalists were seen as something of traitors to the
American cause, they were hated much more than a simple enemy would have been.