The Question and Answer section for Chronicle of a Death Foretold is a great resource to ask questions, find answers, and discuss the novel.
The link below will lead you to a great synopsis by Random House. It is far too detailed for me to give you a synopsis that will do it justice. I've used this site with my own students; it's extremely helpful.
Victoria didn't believe the rumor was serious.... she believed it was nothing more than drunkard's talk. According to Divina, however, Victoria wanted to see him dead.... thus, she didn't warn him.
Santiago, Divina, Victoria, Clotilde, Margot, Luisa, and the murderers are all forewarned about Santiago's death. The fact that we know Santiago will die doesn't matter, as we spend our time waiting for the inevitable..... not to happen.