Saboteur
WordNet
noun
(1) A member of a clandestine subversive organization who tries to help a potential invader
(2) Someone who commits sabotage or deliberately causes wrecks
WiktionaryText
Noun
- A person who intentionally causes the destruction of property in order to hinder the efforts of his/her enemy.
Etymology
sabot + -eur (where mill workers would throw their wooden shoes into the machinery to make it halt or break down.)