The shadow side of any zodiacal sign is connected with the parts of personality which the individual can't recognize and can't express.  And you might expect that with all Capricorn's realism and earthy wisdom and emphasis on the mundane and the practical, all those repressed emotions and dreams and fantasies have to go somewhere.  After all, Capricorn is complex and sensitive, and is much deeper than he often appears.  So we might call his shadow side the Fanatic syndrome.  This particular figure is connected with another one, which we might call End Justifies Means.  The scenario goes something like this:

FRIEND:  I hear you're standing for Parliament.  That's really good news.  I always thought you ought to pursue a career in politics.  What do you see yourself doing?
CAPRICORN:  Well, I have some long-range plans.  If I can get this seat, then I can begin to get my ideas about cleaning up Soho accepted. 
FRIEND:  Oh, come now.  Why do you want to clean up Soho?  I thought you were such a realist. 
CAPRICORN:  (looking stern and patriarchal)  It's about time this country returned to the good old values of our fathers.  All those disgusting pornographic films and prostitutes on every corner and the police can't even control it….
FRIEND:  Oh, surely they don't harm anyone, and besides, it probably fulfills a useful function.  Lots of people need to let out the seedier side of themselves. 
CAPRICORN:  It's total self-indulgence, greed, in fact it's sinful.  Why, it's destroying the family as the basic unit of society.  Young children can go and see those disgusting shops where you can buy all those terrible things, and they're corrupted before they even go to college.  I'd shut every shop and close every cinema.  I'd arrest anyone who…
FRIEND:  I thought we had freedom of speech and press in this country.
CAPRICORN:  But it's for the good of the whole.  Oh, I suppose it would ruin a few people who make their money peddling those revolting things, but in the long run…

The Capricornian shadow gone fanatical may have an ace to grind just about anything.  When you first hear him going on, you might laugh.  Ah, well, silly man or woman, moralizing that way, doesn't he or she know that you can't change human nature?  But the Capricorn shadow thinks you can.  And whether the issue is about sex, or money, or religion, or politics, or just giving you lots of unwelcome advice about how you should run your life, the Capricorn shadow gone out of hand freeze your blood, because it's like Yahweh in the Old Testament:  do as I say and don't question it.  The shadow side of Capricorn isn't necessarily politically conservative, either:  remember that Stalin was a Capricorn too.  What it's really about is that Capricorn, gone over to the shadowy side of his nature, always has a plan to change society, which of course necessitates changing it his way, which of course means there's no room for dissenters, which of course means you either forcibly remove them or pay them off.  Richard Nixon, too, is a Capricorn.  End Justifies Means. 

The thing that needs changing in Capricorn's eyes also needn't be a big section of society.  It might be you:  the husband or wife,