Because, for various reasons, terrorism is the main tactic of Islamic extremists.

The main theory behind terrorism is probably best explained by a Chinese proverb loved by the US Army Snipers - "Kill one, terrorize a thousand"... Islamic extremists aren't often in the position to kill a thousand, but by killing a few indiscriminately they are better able to de-moralize a whole nation. Terrorism isn't about wiping out a population, or wiping out an army. It also is not assasination. It is indiscriminate, yet is still quite targeted, killing with the aims of spreading fear. As a tactic, killing people seemingly indiscriminately works. That's why they do it - it works, and it works better than anything else because whilst we have become almost experts at conventional warfare, we struggle against terrorist tactics.

They believe that any infidels will go into the flames and any good Muslims they kill will achieve paradise. A no-lose proposition for them. +5

there's rarely anything indiscriminate about it. actually, they're usually pretty discriminating about who they kill.

To spread fear, don't know why.

Because in their minds (?) said people don't count as 'people'.