We're not going to be smarter than a superintelligent AI. The things we conceive it doing if it were given a malicious task (bioterrorism, killer nano-machines, pure fusion bombs sidesteping the non-proliferation bottleneck of Pu239, etc) are likely not the full set of things it can do to harm us. I don't think it does us any favors to dismiss the risks here.
Even the things we can conceive of are very scary, to me at least.