Only some explanations on both meanings, "to prey on..." is more like to take

advantage, getting some benefit (money etc) on the misfortune as in:

http://www.fema.gov/news/newsrelease.fema?id=52236

LINCOLN, Neb. -- Survivors recovering from the severe storms, flooding and tornadoes that began in June are urged to beware of scam artists that prey on others’ misfortunes, cautioned state and federal officials.
"Unfortunately, some people take advantage of others when they are at their most vulnerable,” said the Federal Emergency Management Agency’s (FEMA) Stephen R. Thompson. “We remind residents to be especially careful."

Preying on misfortune | James Hultquist-Morrissey SCAMS
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Você marcou isto com +1 publicamente. Desfazer26 Dec 2010 – Scum like James prey on the misfortune of others and promise them the life they once had, doing the kind of work that they are passionate ...

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Você marcou isto com +1 publicamente. DesfazerUnfortunately, there are companies out there that exist simply to prey on others' misfortunes. It is extremely important to compare companies before deciding to ...

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       The other meaning  is "taking pleasure" by the fact of seeing somebody in deep waters (serious problems), or somebody being in worse condition than him.

     In psychology they call it  "Schadenfreude".    Sometimes coming from envy of the way he/she thinks the other(s) deal with his hardships or flaws, there is, it´s not

about having any benefit, it´s about "perception" .

        It´s also a kind of egotism, since in practice is like saying "Since I am not happy, I don´t want anybody happy as well."

Why Are We Pleased With Others' Misfortune?
It is not enough to succeed; others must fail
Published on January 30, 2009 by Aaron Ben-Zeév, Ph.D. in In the Name of Love
It is not enough to succeed; others must fail. (Gore Vidal)

http://en.wikipedia.org/wiki/Schadenfreude
Schadenfreude  (German) is pleasure derived from the misfortunes of others.[1] This German word is used as a loanword in English and some other languages, and has been calqued in Danish and Norwegian as skadefryd and Swedish as skadeglädje.[citation needed]