I currently searched up the question your looking for and this is what I somewhat found
From Middle French massacre"wholesale slaughter, carnage," from Old French macacre, macecle "slaughterhouse, butchery," of unknown origin; perhaps related to Latin macellum "provisions store, butcher shop."
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Answer:
1. Them being for the sake of their economy, religion and glory.
2. They wanted to improve their economy for instance by acquiring more spices, gold, and better and faster trading routes.
3. they really believed in the need to spread their religion, Christianity.