Let's reword the question. How ethical is it for the Witnesses to go around condemning value creators? How ethical is it for the Witnesses to go around trying to get people to give up their lives and waste it promoting a scam?
First, they make a promise of everlasting life, intentionally leaving out that it is going to be full of stagnation and order-following. Then they make a lot of rules to get there--the first one is not looking up alternate sources for cross-checking the accuracy of the materials. You are supposed to check it using only material that has been supplied to support the doctrine. And then they keep hounding you to do ever more--go to indoctrination sessions, join the Theocraptic Misery "School", go out in service, get baptized--
And from there, they start adding workloads. None of that was told when you first received the message. They never told you that putting in 15 or 20 hours a month was not going to be enough. They want people to pioneer. They want people to give up college--I never saw that one in the study publication. They want people to work part time and give up material comforts (and as Danny Haszard can attest to, often necessities as well) to do more in the field misery. That was never in the study material, either! And there is the constant insinuation that it's "Pioneer--Or Die!!". That was never put in the showcase material, either.
I believe that it is OK to plant seeds of doubt in sincere Witnesses, as long as you are honest about what is going on or what you see is plausible in the future. Telling the people the real history of the cult, the fact that the Bible has been deliberately altered by people with little or no knowledge of Hebrew or Greek, and what is developing is perfectly ethical. It is also ethical if you think, based on what you are observing, to make plausible predictions and state where you feel the organization may be heading. And, especially, encouraging them to do some more independent research about the organization can help them form their own opinions about where it's headed, and allow them the tools to make a fully integrated decision as to whether they wish to stay and be fleeced or leave.