In an attempt to uphold the integrity of the Law, the 40 Jews were willing to desecrate the Law in order to stop Paul. They took the oath of fasting, and instead of focusing on God, they focus on killing Paul. Do I make up my mind and do things my way, for my purpose, instead of focusing on God and His will? The Jews' zeal was based on upholding God's law, but that was only used as a surface excuse, they had given up seeking God's will and began to seek their own.
Although Jesus had promised Paul that he would make it to Rome, when his nephew came to him with news of the plot, he sent his nephew to tell the news to the Commander. Just because we have a promise from God, it does not mean we sit back and not participate in the events of our life.