Contrary to what Steve-O said, there is nothing indicating that the effect stops somewhere during your turn.
Nor does it say that he is under your control for the entirety of your tun. Interpreting the rules loosely for your own purposes is a slippery slope. The excuse "It doesn't say I can't" is never a good one when interpreting rules, and is almost always an incorrect interpretation.
It says to treat the hero as a monster this turn
No, it doesn't. You've taken two parts of the sentence that are not together and put them together. You can't just rearrange sentence structure without possibly changing the meaning.
It says "During this turn, you may perform 1 move action and 1 attack action with this hero, treating him as a monster in his own monster group." It doesn't say that you treat him as a monster in his own monster group for all intents and purposes of what a monster is (picking up special quest tokens, opening doors, etc.). Both the first part ("During this turn") and the last part ("treating him as a monster in his own monster group") are most likely modifiers to the middle part of the sentence ("you may perform 1 move action and 1 attack action"). As such, those two very specific things you can do with the hero must be done during this turn and when performing those two very specific things you can do with the hero, he is treated as a monster in his own monster group. This means the attack must target a hero and not a monster, his movement can not move through other heroes but can move through other monsters, and also provides clarification for how to treat him when using cards and abilities which refer to monster groups.
That said, it is possible that this interpretation is not what FFG intended (they have done things like this in the past where the correct interpretation of the rule is not to treat the rule strictly as written), but in this case, I HIGHLY doubt it because an immediate defeat to a hero is incredibly overpowered.
IMO, if it was intended the other way, it would just leave out the middle part and say "During this turn, treat the hero as a monster in his own monster group" since it's already in the rules that all monsters only get one attack and one movement, and if they wanted to add that as clarification, it would have come at the end of the sentence (as "treat the hero as a monster in his own monster group" does on the card as written).
Edited by griton, 29 October 2013 - 09:18 AM.