Can I take a tune of a Christian song and change the lyrics to make it Jewish? Is it permitted being that it is a mitzvah to turn a church into a synagogue?

If one has to take a job that involves doing something that could lead to deaths (e.g. manufacturing bombs for a country not yet at war, but talking about it) in order to earn enough money to save your brother's life, is it ethically acceptable to take this work? (Situation in a novel I'm writing.)