What can I do, if I don't like the proposed research subject by my potential supervisor?
A successful project profits from both supervisor and candidate being interested in the same question. If you really dislike the topic, you should try to convince the supervisor to accept a different question. He will then, if he is amenable, try to convince you of a variation of his, you will amend it, etc. until - hopefully - convergence.

However, you may find that the supervisor is simply not interested in your direction. This can happen, even if he likes you. That's fine, and then you either have to find somebody closer to your question of interest or compromise.

That being said, it seems that you are talking about an MA Sc degree. This is considerably shorter than a PhD, so, depending on the details of the situation, it may be easier for you to find a compromise.

If topic really matters for you, however, you need to think hard whether you want to go into a field which you have qualms in. 

I am not sure about what the ethical consideration of a "theoretical topic" as opposed to a practical one could possibly be; be is as it may, I would avoid bringing this up, as it may insinuate that you cast doubts about the ethics of your prospective supervisor.