The Collaborative International Dictionary of English v.0.48:

Looter \Loot"er\ (l[=oo]t"[~e]r), n.
   A plunderer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

looter
    n 1: someone who takes spoils or plunder (as in war) [syn:
         plunderer, pillager, looter, spoiler, despoiler,
         raider, freebooter]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "looter":
   depredator, despoiler, forager, forayer, freebooter, marauder,
   pillager, plunderer, raider, rapparee, ravager, ravisher, reiver,
   rifler, sacker, spoiler, spoliator, wrecker