The marginal increase in the probability of an someone being a terrorist given that you know he's an engineer may be startling in relative terms, but in absolute terms it's insignificant.
Estimates of total active membership in terror groups worldwide is under 200,000, but let's assume there's even million active terrorists just for the sake of having round numbers and not having to quibble over where to put the decimal point. There are seven billion people in the world, so the rate of terrorist participation in the general population is 14 thousandths of a percent; let's call that p(T), and call the probability that someone is a terrorist given that they're an engineer p(T|E). Let's look at the absolute marginal difference being an engineer makes, i.e.:P(T|E) - P(T)
i. p(T) = 0.0001428
i. p(T|E) = 9 * P(T) = 0.001286
iii. P(T|E) - P(T) = 0.001143
So being an engineer increases your chance of being a terrorist by at most about 1/10 of 1% under wildly pessimistic assumptions. In fact the marginal difference is really more like 1/50 of 1%. Now it's interesting that the rates of terrorism are so much larger among engineers than other people, but it has little practical significance and being an engineer myself that's what I'm most concerned with. If you were designing a surveillance program and were picking out groups that need keeping tabs on, 1/10 % is a grasping-at-straws number