I think the choice of process is basically orthogonal. Waterfall and Agile processes done well both have been demonstrated to produce safety-critical software. (See, e.g., Nancy van Schooenderwoert's work in medical devices.) Both can produce dangerous software. (E.g., Toyota's "unintended acceleration" issues.) The question is much more about what practices you put in place to assure quality.

I do think short-cycle methods have some intrinsic advantages, though. By making critical functionality available much earlier for testing, you get more time and more chances to make sure it's really safe. If early versions are bad, you get early warning signs that aren't available in a Waterfall process. And if testing turns up issues early on, it's much cheaper to make fundamental architectural changes: less code to change and more time to change it in. And Waterfall processes aren't at good at dealing with unknown unknowns; some things you only learn by trying them out.