It's a good idea, but I don't see it happening. If I'm understanding what you are saying, then the ability is correctly exemplified below:
bob has reaper on the field and jenny has a massive dragon on her field
bob uses reaper's ability and massive dragon goes *poof* and disappears
reaper now has 30 poison counters
Please note that the word "reaper" could refer to either the upgraded or unupgraded version as it makes no difference.
So if I'm correct, the quintessence of this ability is instakill?