Well, these days I solve problems for a pharmaceutical company, and before that, I was a consultant, but before that, I spent 15 years in the Nav, most of it either running nuclear reactors, or repairing them (and associated systems).
It would be tremendously difficult to create a large enough quantity of Co-60 to produce anything more than a few enhanced-fallout weapons. Even then, you’d need to be constantly replacing the material, as Co-60 has a relatively short half-life, at 5.27 years. I can’t see any nation bothering anyway, as you can get nearly the same effect by employing ground-bursts, at the cost of using a special weapon in a less-than-efficient manner.
As for other means of wiping-out human life, well, I can only think of a few, and short of meteorites, the best shot is biological warfare. You’d need a lot of your chosen agents, and you’d probably need a number of different agents, so that if some number of people turn out immune to one, another would get them. You’d need to have an agent that was very persistant, highly infectious, and capable of long-term survival outside the human body. You’d have to seed the stratosphere with massive amounts of your agents to get global coverage, which means that whatever nasties you choose will need to be UV resistant, too. Pretty tough mission requirements, if you ask me.
Even if you could meet the mission parameters, I wouldn’t bet on 100% success. Humanity exists on this planet in very isolated places, and is remarkably tough.
The next best choice would be to pollute ourselves to death. It’ll take longer, but can be done more thoroughly.
Gah, enough grimness for one day.