How'd we get from free-and easy to fatal accident!? Sure, I know, you can explain it. But isn't it more fun not to?
Casual and casualty are derived from the Latin casualis, "by chance". So people who act casually may do things unpremeditatively and casualties died by chance. So it makes sense. At least it makes some sense that the root took on wildly dissimilar denotations. What still doesn't fit is how "ty" changed everything. Take the next example:
Sanguine: confidently optimistic and cheerful
Sanguinary: gory: accompanied by bloodshed; "this bitter and sanguinary war"
Whoa, that turned sour fast! What message are we sending here? It's a short trip from confident, blithe, and easy-go-lucky to a gory end? Both words come from the latin root sanguineus, "of blood," so sanguinary is self-explanatory. Sanguine comes from a theory in medieval physiology that believed in four "humors", black bile, yellow bile, phlegm, and blood. It was thought that an excess of blood contributed to a hopeful and confident, or sanguine, personality. My question is who decided that "ne" is the good one and "nary" is the bad one?
Well, nary does mean "not a" as in "nary a whisper was heard" and according to the Australian government's Noisy Scrub-bird (Atrichornis clamosus) Recovery Plan, Ne stands for "effective population size; the reproductively-active part of a population," so there's something.
I ought to be more controversial.
definitions from WordNet®
etymology from Online Etymology Dictionary