However, I don't think that it is outside the realm of possibility that relatively small entities, on the order of a small-cap corporation or private laboratory, could eventually create a weapon that has far more global destructive power than a radioactive dirty bomb.

This would afford the entity more leverage at a global bargaining table- as opposed to a negotiation with a single government. Combined with the relative anonymity one may be able to maintain when releasing the virus, these entities might see a higher expected value in engineering a virus.

It would be "DIY" in the sense that you wouldn't need the backing of a large nation state to produce an ICBM, or opportunistically steal some radioactive material to create a dirty bomb.

Also, a virus is implicitly cheaper to distribute: with a longer incubation period and an exponentially increasing rate of propagation, a virus could reach critical mass before there's time to react.

In a sense, this makes them more effective than other types of WMD- which may have lower singular chances of successful detonation, have high per unit capital costs, tend to have local area effects only, and are generally difficult to produce far removed from military industry at scale.

Personally, I think the world is more likely to end with a bug than a bang. I honestly don't know which is more terrifying to me.