Zero-days and backdoor access points are like turn-key WMDs and the rest are analogous to small pistols and oddly shaped special-purpose wrenches. As long as the zero-days and backdoors stay private, I think there isn't that much concern.
If they don't stay private, then, yeah, it's a severe risk to the world. But those aren't exactly tools; more like privileged knowledge.
You're unlikely to find some super secret intelligence agency hash cracker or DDoS tool that's 100x better than all existing free and paid tools, or something like that. And I think you're probably not going to find a leak that gives you access to a gargantuan government botnet, as you suggest. Even if they're incompetent enough to somehow have that become exposed and accessible by some random internet person, they'd almost certainly shut it down within minutes. Also, odds are such a person could rent a larger botnet through a number of much simple means, anyway.