Fortune does not want to kill Lucius, she just wants to toy with him, much like cats seem to do with mice, almost to the point of trying to test him, for whatever reason. It could be that Fortune and Isis work together in this, though this is pure speculation. Fortune also seems to be a divine or supernatural personification of what might be fate in the Aeneid , although, unlike fate, Fortune can be thwarted. Mortals can surmount Fortune at times, but never completely conquer her. However, fate can never be overcome.
Mortals have no influence over fate and neither do the immortals.
I think that this weakness of Fortune is of primary importance in the novel. Because Fortune can be defeated by the divine, and only by the divine, it is necessary for mortals to appeal for divine grace in order to free themselves from Fortune's grip. It is this divine grace that is the source of security in The Golden Ass . Because Fortune is the root of insecurity, Isis can have a special role in Lucius' life. She is able to keep Fortune away from her chosen.