If the goal of terrorist organizations was to kill the most people, they would target critical infrastructure, like drinking water reservoirs or perhaps air traffic control towers, if they have a thing for airplanes.

They will say they are trying to kill people to help their cause, but it's more likely that they operate as any other 'business'. They try to impress the 'investors' (e.g. Wahhabis with oil money), so they can get more resources and expand into other 'markets'.