For argument's sake let us start a list.
Things that, although you volunteered to do,
would eventually drive you mad. Adopting
cats which you know had eaten their owner
shall top the list. 

 At first a honeymoon of mewing and purring.
But how soon would it be before you entered
a room and found one of the cats peering at
you from a doorway. With a conspiratorial
glance it would gather up it's familiars and
blow away like dry leaves to a dark spot in
a vacant bedroom. Where, if you follow them,
you will see 2 forepaws described at the
edge of a shadow by the bed. A solar system
of eyes wise enough to know that you can see
them even in the darkness in which they
float inspect you. 

The eyes weigh you,
waiting with the patience of a natural
order, for you to weaken. A cue, one you
might miss, will remind them just how
vulnerable you are. How thin your skin is.
How supple your flesh. The pleasure of
burrowing a head in your stomach cavity, of
batting at a half torn nose, a mouse on a
string. It would be so easy, and so easy to
retain their innocence afterwards. 

Another owner would come forward.
Other, easier to own cats, would have to 
be destroyed that week. But these with 
their story would have
adoptive cat owners lined up to shelter
them. For after all, it could not have been
their faults. They would plot more quickly
the next time. Brazened, encouraged by the
response to the other murders. 

Monsters of man's own creation weeding 
out from society
those responsible for their creation.