Source : Webster's Revised Unabridged Dictionary (1913)

Undoer \Un*do"er\, n.
   One who undoes anything; especially, one who ruins another.

Source : WordNet®

undoer
     n 1: a seducer who ruins a woman; "she awoke in the arms of her
          cruel undoer"
     2: a person who unfastens or unwraps or opens; "children are
        talented undoers of their shoelaces" [syn: {opener}, {unfastener},
         {untier}]
     3: a person who destroys or ruins or lays waste to; "a
        destroyer of the environment"; "jealousy was his undoer";
        "uprooters of gravestones" [syn: {destroyer}, {ruiner}, {waster},
         {uprooter}]