Because Operation Red Sea is a fictional film, the plot and characters' behaviors are fictional. However, based on the plot and dialogue of the film, I can provide the following explanation:

In the movie, the old man with glasses is one of the parties to the yellow cake trade. He received a phone call, and the person on the phone provided him with information about the rebel leader, suggesting that the rebel leader was betraying him. Therefore, he decided to let his men kill the rebel leader to protect his own interests and safety.

It should be noted that this explanation is only a possible speculation based on the movie plot. We can't be sure what the phone call is, and we can't be sure whether the old man with glasses really received such a phone call. So this explanation is for reference only. If there is more accurate information or official explanation, please refer to the actual situation.