IMO, existential paranoia makes sense in the same way it makes sense for an engineer to be paranoid about a bridge, plane, or nuclear power plant they are building: Lives are at stake and there's no "redo" button if you don't get it right the first time.

It's not the prediction of a sensible theory of evolution. It has nothing whatsoever to do with evolution, and I struggle to figure out where the idea that it does comes from. The idea, correct or incorrect, is the result of the extrapolation of several, independent trends (in particular, nanotech and AI). We've managed not to kill ourselves so far, but that's partly a matter of luck. Even if the only way we could kill ourselves was with nuclear weapons, there's still a nontrivial chance that we would. Especially with India and Pakistan in on the game now. And there are new threats as well.

Edit: I don't necessarily think that existential disaster is more probable than not, but I definitely think it shouldn't be dismissed out of hand. And since people are downvoting this, I'm wondering where they disagree with that.