Bioterrorism strongly depends on the incubation period. Probably safe on Mars. Rogue nanotech - not unless it has AI. (Though gray goo is not considered very realistic nowadays, as we've realized just how hard even regular nanotech is, and how hostile the natural environment is to nano-scale mechanisms.)

Nuclear war is the big one. Mars colonization would be less about preserving human life, per se, and more about preserving an operational duplicate of the peak state of human civilization.