Swansont's answer is basically right. If the mad scientist somehow had a few kilograms of positrons captured in a magnetic bottle, and then let them go, the energy released would be quite large, and there would be lots of non-radioactive fallout. There would also be a lot of corrupted molecules potentially creating dead or mutated life, and or biochemical toxins. It is also possible that enough energy could be released that some nuclear interactions could happen in the shock wave as to cause some strange short-lived isotopes, but probably not many.
If on the other hand, your mad scientist released several kilograms of solid (very cold) anti-Hydrogen. The gamma rays released would be enough to alter the nuclei of the atoms nearby, and create a nasty soup of strange short-lived elements in the material blasted up into the atmosphere (from which it would later fall out).
Forming opinions as we speak