First, Remember it was written in 1792 for the army of the French revolution. I'm French, I've heard a lot of of explanations about these words since I'm a child. Here's one of them: https://fr.quora.com/La-Marseillaise-contient-Quun-sang-impu.... Sorry for the French text.

This is a question that is sometimes very much discussed in France. Always in French : https://www.uneautremarseillaisepourlafrance.fr/blog/2015/12...