A hatchet job is defined as "a very strong and unfair criticism, especially in the form of a newspaper article". That's a rather mild definition. I'd call it a deliberate attempt to harm or destroy in a news media article.
In an attempt to recover his dismal ratings, Anderson Cooper, that handsome CNN presenter (I won't call him journalist as that would insult real members of the profession), did a hatchet job recently on Scientology . The facts of what happened may fascinating reading. You can read all about it (and see some really good videos) here: CNN The Least Trusted Name in News - AC360: See Anderson Cooper Run for Ratings