Someone killing a hundred people who would gladly kill a hundred million if they could, and who's presumably constantly looking for a way to do it, is more frightening than a car accident, because there can't be a huge sudden increase in the rate of car accidents, but there can be and there more than once was a huge sudden increase in the amount of people killed by enemy combatants.
(Whether you think terror could ever escalate into a full-blown war is another question, and many don't think that, or they think that it is the common countermeasures against terror that result in full-blown wars rather than terror itself, etc. My only point is if it's legitimate to fear paperclip maximizers because they're "strong AI" and always achieve their goals, it should be legitimate to fear dead-people-like-you maximizers because they're a form of wetware intelligence with a good track record of achieving its goals, and they specifically want to kill you, rather than it being a likely subgoal of "make more paperclips". From a citizen's point of view, the state is equally powerless against terror and car accidents, and still the dead-people-like-you maximizer is way more frightening for many and it's not stupid to think that.)
The scariest link there - I've said this before, but there is no harm in repeating it - is that Pakistan probably has both, we're sure about their nukes and we strongly suspect they have a BW unit.
IS gaining a foothold in Pakistan gives them potential access to weapons of a totally different grade than the ones that they have today.
If that should happen and these weapons would be brought to a readiness level associated with an imminent attack or if - which I hope we'll be able to avoid but which you can't rule out entirely - an attack with such weapons would be carried out the degree and scope of response will be such that the map of the regions involved will be totally redrawn.
As soon as terrorists cross the line where all-out war against them is justified and the cost to society is one we are willing to bear (and we're far from that, right now the most people can be seen to support is to drop some bombs from a high altitude) it will be 'game over' in a relatively short time with respect to the first level of confrontation and after that we will have a repeat of the aftermath of the Iraq war but on a much larger chunk of the globe.
The world is a tinderbox and way too many idiots have matches.
Replace 'enemy combatants' with 'comets'.
Do you still find your argument persuasive?
In seriousness - actually comets are a lot like car accidents (if you believe that comets aren't actively trying to kill you), the only difference is the probability distribution (and you used the comet example because of the higher variance.)
Unlike those things, the success of an intelligent being trying to kill you cannot and should not be thought of as a probability distribution. It's not like a coin being tossed, in that a coin doesn't tend to get better at falling on one side or the other (nor do car accidents or comets get better at killing you, so they are more like coin tosses.)
It matters whether there's intelligence behind the cause or not; I'm surprised at how hard this point is to make at a forum occupied mostly by programmers.
Funny words: full-blown war scares me much more than mere "terror".