Final answer:
Aetes didn't kill Jason at first because Jason arrived in Colchis seeking the Golden Fleece and Aetes challenged him to complete tasks in order to obtain it. Jason's success in completing these tasks with the help of Medea prevented Aetes from killing him without potentially provoking a conflict.
Step-by-step explanation:
Aetes didn't kill Jason at first because Jason arrived in Colchis seeking the Golden Fleece, which was in Aetes' possession. In order to obtain the Golden Fleece, Aetes challenged Jason to complete a series of tasks, including plowing a field with fire-breathing oxen, sowing dragon's teeth, and defeating the warriors that sprouted from the teeth. Aetes hoped that these tasks would be impossible for Jason to complete, and he wouldn't have to give up the Golden Fleece. However, Jason was aided by Medea, Aetes' daughter, who fell in love with him and helped him successfully complete the tasks. As a result, Aetes couldn't kill Jason without angering Medea and potentially provoking a conflict with his allies, as Jason was a renowned hero with influential backers.