Bottom Feeder Meaning

(idiomatic, by extension) A person who operates amidst or thrives on the unwholesome things in a society; one who takes advantage of the misfortune of others.

Example: A fish or other aquatic creature that feeds off the bottom of its habitat; a flatfish.
2013 Nov. 16, Christian Dem in NC, "Bottom-feeders using social media to intimidate witnesses," The Daily Kos:
  â€œ[T]he rise of Facebook, Twitter and other social media has made it harder to control the flow of information, thus making it easier for bottom-feeders to bully witnesses.”