In the book it was just a random farm scythe. Stronger element of JUSTICE in the movie version, for Santiago to be killed by his own – and very real, not a prop – scythe.
“In a kitchen garden I saw something, something that had only been vague in my thoughts until I had my hands on it. It was a small scythe, its sharp curved blade still caked with green weeds from the last mowing. And once I’d wiped it clean and run my finger along the sharp blade, it was as if my plan came clear to me and I could move fast to my other errands…” – Louis de Pointe du Lac, Interview with the Vampire
I-want-my-IWTV 