Why have it take one specific Hex, rather than simply do an attack? Because it lets them draw a card first, there's no worry about stacking too much, so you don't need the "at least 5 cards in hand" part of Fear.
Witch Hunter - Action-Attack - $5
+2 Cards
+1 Action
Each other player draws a card, then reveals their hand. If a player reveals a card with the word "Curse" in its text or types, he or she discards an action or treasure.
This does almost the exact same thing, with fewer words.
I still think this will help an opponent far more than it will hurt them. In a worst-case situation, you could leave your opponent with 5 Curses in hand every turn (and using your original version with Fear doesn't help that problem).
So as a whole, I don't get this card. It most situations, it's just a far worse Laboratory, just a lab that also gives your opponents a free draw. But in rare worst-case situations, it's a way to lock your opponents out of the game forever. There's almost no situations where it's a fun or balanced card.