Septimus: Be careful with the flame. Thomasina: I will wait for you to come. Septimus: I cannot. Thomasina: You may. Septimus: I may not. Thomasina: You must. Septimus: I will not. (2.7)
This interaction between Septimus and Thomasina suggest that, regardless of where the chips fall on fate vs. free will, there are some things that humans think they control, and others that just happen. Septimus's refusal to go up to Thomasina's room is an example of the first kind of thing, and Thomasina's death by fire is the second (unless you think she committed suicide). The different meanings of "will," "can," "may," and "must" show just how difficult it is to pin down where to draw the line between the two. If Septimus had chosen to follow Thomasina, would she have lived? We'll never know.