Given a modest budget we all had to come up with a plan to destabilize society, 9/11 style attacks, whilst spectacular eventually don't really do a lot of damage, are costly and failure prone though they can definitely drive policy changes and result in a nation doing a lot of harm to itself it will ultimately survive. But what if your goal wasn't to create some media friendly attack but an actual disaster instead, what would it take.
The stories from that night continue to haunt me today. My own solution led to a lot of people going quiet for a bit and contemplating what they could do to defend against it and they realized there wasn't much that they could do, millions if not tens of millions of people would likely die and the budget was under a few hundred bucks. Knowledge about technology is all it takes to do real damage, that, coupled with a lack of restraint and compassion.
The resources are indeed asymmetrical: you need next to nothing to create mass havoc. Case in point: the Kennedy assassination changed the world and the bullet cost a couple of bucks assuming the shooter already had the rifle, and if they didn't it would increase the cost only a tiny little bit.
And you can do far, far worse than that for an extremely modest budget.