Not one of the things you mention is easy to accomplish at the scale it kills tens of thousands. The sheer volume and complexity of distribution of explosives

Not one of the things you mention is easy to accomplish at the scale it kills tens of thousands. The sheer volume and complexity of distribution of explosives or gas needed, or the rarity and oversight of biological agents necessary renders those attacks anything but simple. You're talking dozens of people coordinating against hardened and watched targets with either a substantial planning period or access to high volumes of controlled substances. Nothing at all about a few pissed off or crazy people or easy.