Is radioactivity good or bad?
Radioactivity as it is applied to nuclear medicine and to sources for radiography and the like are, in general, good. (In light of the true sense of the question, nuclear power generating plants are being set aside. They are about heat and power generation, and “radioactivity” is a by broduct.) There are broad applications for this technology and some insight into them can be had by reviewing the Wikipedia articles. (Links are provided.) There is a down side to radioactivity, however. Uncontrolled radiation poses a grave risk, and one must do a cost-benefit ratio. Additional difficulties in assessing the down side are associated with the fact that some radiation is extremely long-lived (thousands of years!), and, of course, radiation is invisible and can “sneak up” on us in certain situations. Additional links are provided.
