I think the wording of the healing part could be made a little clearer. Maybe something like:
"Weapon : deals 2 damage at the end of every turn.
Heals 1 hp for every poison counter on a creature in play."
As far as I can tell it should be pretty well balanced. It potentially allows some of the highest healing per turn in the game. However, since most creatures will only withstand more than one or two poison for a few turns before dying, it should be difficult enough to abuse.
Plague decks are obvious candidates for this card, although it may be possible to pull off a fun voodoo or even voodoopult type deck as well.
Just how much healing you will get out of one of these on average is actually very tricky to predict. I think play testing will probably be needed before we can really know for sure.
One odd question, how will purify status interact? Will it be subtracted or ignored ... or added as if it were regular poison? I can see pros and cons to whichever way you go.