I agree with you that Fish and Chikatilo certainly are standouts among the many. Two other favorites are those hillbilly booger-eatin' morons Otis Toole and Henry Lee Lucas. Too good to be true. Cartoons!
If you're asking why I find many psychopaths pathetic it's because they seem to me to be enslaved by the characteristics of their personalities in a much more rigid manner than the average Joe. Suffering from the delusion of freedom but chained by their consuming impulses.
How many of them exist among the population of the US is questionable but guesstimates are, a probably modest, five hundred.
Since a true psychopath tends to escalate his behavior with each "success" and because the behavior has, (thus far) been so far outside the scope of accepted human behavior, most seem doomed to escalate themselves into self-destruction or imprisonment. And sadder yet, never seeming to achieve satiation. The best the best of them can manage is mention in a psychiatric handbook.
But how gnawing that endless hole in the soul must feel to the person who has it. Tortured individuals.
A frenzy of malice expressed in narcisscistic satisfaction which consumes its very host. The inevitable sordid ending of the ones we know makes them figures of personal tragedy for me.
"Evil" is a greedy master which devours all it reigns over. My take.
But, oh, the ride must be glorious madness while it lasts and therein lies the fascination. For me it has become a story with the inevitable similar ending, the sole interest dwelling in the detail.