bottom feeder
1. Someone who occupies or advances him- or herself from the lowest position in society, politics, business, etc., by taking advantage of the misfortunes or dregs of others. Likened to fish or other animals that feed on refuse at the bottom of a body of water. Money lenders are often considered the bottom feeders of the financial world. Political pundits are all just bottom feeders in my eyes.
2. Any person of low, despicable, or dubious social status. This is an ugly part of town; it's full of drug dealers and bottom feeders. Jerry's just a bottom feeder: he won't get a job, so he just mooches off us every day.