The Marauders were gathered by Gambit at the behest of Mister Sinister, creating a team of mercenaries and assassins to do the mad geneticist's bidding. They were specifically formed with the slaughter of the Morlocks in mind, and carried out the Mutant Massacre as their first mission. By sampling their DNA for his records, Sinister has made the original Marauders virtually immortal thanks to his cloning technology. With the exception of Sabretooth and the psychic entity Malice, none of the original members (Vertigo, Scalphunter, Arclight, Riptide, Blockbuster, Harpoon, Prism and Scrambler) is still in their original body.
Polaris and Omega Sentinel were never willing members of the Marauders -- they only served as long-running host bodies for Malice.