I like this idea, but it presents a terrible problem with cards like Ball Lightning and Virus which can be killed at will; Fractaling these creatures would result in terrible amounts of damage, too terrible in my opinion. I think it could work if the damage was reduced to a lower amount.
to be honest even if it was reduced to 1 damage per creature dying, it would still be OP with BL fractal, think about it, with 10 BLs you do 50 damage + 10 damage = 60 damage.
upgraded version would have to be 2 damage each creature, which would make 70 damage.
I actually think this would work better if the ability was the following:
Add one poison counter to the opponents Hp every time a creature dies (non-stackable)
that would give a good buff to poison but not necessarily an OP buff.