Slavers play a fairly large role in Fallout 3. Paradise Falls is a slaver compound, and several quests feature slavery as either major or minor elements, most notably Head of State and its association to slavery and Abraham Lincoln's role as the 'Great Emancipator.' Additionally, the Lone Wanderer can capture people to sell them to slavers. The Lone Wanderer can even buy a slave, Clover, who will then serve the player as a bodyguard. The player does not have the option of activating the explosive collar of Clover. The number of slaves present compared to the number of slavers is remarkably low, meaning that the slaves are being shipped off somewhere. It is revealed by Wernher that many of these slaves are being shipped off to work at The Pitt in the DLC of the same name.
If you start bringing slaves to Paradise Falls you may trigger a random encounter with a group of slavers that can't keep up with the number of slaves they have to bring to Eulogy Jones, so they try to kill you to eliminate the competition.
Slavers are generally not hostile to the player as long as you don't harm them or provoke them with hostile dialogue responses, however attacking them can make the Paradise Falls slavers permanently turn against the player's character. The exception to this is that a few jealous slavers may randomly attack the Lone Wanderer for making them look bad. In this case the Paradise Falls slavers will not act as if they ever heard of the slavers being killed.