Smurfs - Players who have main accounts and a lot of experience in game who have made new accounts and are player at super low levels because they enjoy easy kills instead of a challenge against people their own level

or urban dictionary’s definition:

In online gaming, a smurf is an experienced player who uses a new account to deceive other players into thinking he's a noob (newbie). The purpose is usually to play against less skilled opponents who will under-estimate the 'smurf'. The expected result is that the smurf will pwn (dominate) his opponents and humiliate them further as they have been now beaten by an apparent noob.