bottom feeder

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A fish or other animal that feeds on the bottom of a body of water.
2. n. One that feeds low on the food chain; a scavenger.
3. n. Slang An opportunist who profits from the misfortunes of others: "The frazzled, adrenaline-pumped tabloid newshounds [in the movie] are the bottom feeders of contemporary journalism” ( Entertainment Weekly).
4. n. Slang A low or despicable person.

Wiktionary

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

WordNet 3.0

1. n. an opportunist who profits from the misfortunes of others