If a person is about to do a sin and he says Bismillah, has he nullified his Islam? Like he is about to watch pron & masturbate, and he says Bismillah, has he nullified his Islam?
Saying bismillah before sinning is an act of apostasy to some scholars as they say that a person saying this is asking Allah to facilitate for him in such a sin, which is mocking Allah.
Usually, people say it without intending it. If they know what they are saying over such haram things, this could easily mount to mocking Islam.