Final answer:
Santiago's fiancée rejects him on the morning of his murder due to concern for his safety and fear of the Vicario brothers. She wants to protect him from harm.
Step-by-step explanation:
Santiago's fiancée rejects him on the morning of his murder because she is concerned about his safety and fears for his life. She knows that Santiago is being pursued by the Vicario brothers, who are seeking revenge for their sister's honor. She wants to protect Santiago and believes that by rejecting him, she can prevent any harm from coming to him.