bottom feeder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fish or other animal that feeds on the bottom of a body of water.
• noun One that feeds low on the food chain; a scavenger.
• noun An opportunist who profits from the misfortunes of others.
• noun A low or despicable person.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A fish or other aquatic creature who feeds off the bottom of their habitat; a flatfish
• noun By extension A person who dwells amidst or thrives off of the unwholesome things in a society

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an opportunist who profits from the misfortunes of others