The meaning of this sentence is dependent on the paragraph that comes before it in Mary Shelley's Frankenstein. The speaker had the experience of being disheartened by hearing conflicting theories of electricity, which led the speaker to lose faith in the possibility of science to ever be ultimately true in the way they desired. So, the speaker decided to pursue pure mathematics. In this context, the quote means: "I was taught (by "guardian angels") to associate studying electricity and science with evil, and studying math (or anything else) with happiness." In the quote, "prosecution" is an older use meaning "pursuit", or "practice". The idea is that the monster would be born from the scientist's further studies in electricity, so what guardian angel or divine force that might have been acting on him was trying to subtly shape his will to pursue electricity as a scientific study.