An Irish friend this weekend mentioned that when she was younger she met several members of the IRA, who were immediately recognisable, she noted for their 'coldness and arrogance'. These two characteristics I suspect, are what you need to make a terrorist. For while millions have grievances, often legitimate grievances, it takes a specific type to turn that grievance into a plan to execute in cold blood civilians on a large scale. The numbers, in fact, are very small, though their impact immense.
On the news the terms 'militant' and 'terrorist' have been used interchangeably. As George Szirtes points out,
People who deliberately focus on civilians are simply murderers. If they do so for a political purpose they are terrorists and murderers. Not militants. Not an army. They are murderers with a vastly inflated opinion of their own honour and righteousness. which also makes them hypocrites.