According to Turbine's most recently highlighted The Lord of the Rings Online Lorebook entry, even Ents can turn to the proverbial dark side. One might think that even if they did, it would take them such a long time to work out how to kill you that they wouldn't be much of a threat. Unfortunately, you'll hear a different story from anyone who's faced the dour-root -- or Huorn, as some call it -- in Agamaur.
To be more specific, the monster is an Elite Master boss encounter in the Lone-lands' Garth Agarwen instance. It's probably not the first killer tree encountered by most LotRO players. That honor most likely goes to the Warped Oaks in the Old Forest.