The Gemara tells us that one may discuss Torah in a bathhouse in order to prevent someone from transgressing an issur. Although preventing an issur is admirable, why would it not be considered a ‘mitzva habaa baveira’ since the medium being used here is forbidden?
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It’s not that preventing someone from an Issur is precedes the Issur of talking Torah. That kind of talking is Muttar. It is not considered teaching or discussing Torah. You are merely telling the person not to do whatever it is that is the problem.