Except for one problem...

Article 3 of Convention 1 of the Geneva Convention:

In the case of armed conflict not of an international character occurring in the territory of one of the High Contracting Parties, each Party to the conflict shall be bound to apply, as a minimum, the following provisions:

• (1) Persons taking no active part in the hostilities, including members of armed forces who have laid down their arms and those placed hors de combat by sickness, wounds, detention, or any other cause, shall in all circumstances be treated humanely, without any adverse distinction founded on race, colour, religion or faith, sex, birth or wealth, or any other similar criteria. To this end, the following acts are and shall remain prohibited at any time and in any place whatsoever with respect to the above-mentioned persons:

• (a) violence to life and person, in particular murder of all kinds, mutilation, cruel treatment and torture;

The 'soldiers' do this...

• (b) taking of hostages;

...and this...

• (c) outrages upon personal dignity, in particular humiliating and degrading treatment;

... and this...

• (d) the passing of sentences and the carrying out of executions without previous judgement pronounced by a regularly constituted court, affording all the judicial guarantees which are recognized as indispensable by civilized peoples.

...and this...

• (2) The wounded and sick shall be collected and cared for.

...and they dont do this.

This makes them illegal combatants and thus, as they are not soldiers or 'insurgents' (which is synonymous with rebel which refers to a resisting force against an occupying power- which also happens to be convered provided that they are sanctioned by a government of some type), the only name left for them is terrorists, which is what they are.

Here is the definition of a legal 'resistance movment'

Art 13. The present Convention shall apply to the wounded, sick and shipwrecked at sea belonging to the following categories:

• (1) Members of the armed forces of a Party to the conflict, as well as members of militias or volunteer corps forming part of such armed forces.

• (2) Members of other militias and members of other volunteer corps, including those of organized resistance movements, belonging to a Party to the conflict and operating in or outside their own territory, even if this territory is occupied, provided that such militias or volunteer corps, including such organized resistance movements, fulfil the following conditions:

• (a) that of being commanded by a person responsible for his subordinates;

• (b) that of having a fixed distinctive sign recognizable at a distance;

• (c) that of carrying arms openly;

• (d) that of conducting their operations in accordance with the laws and customs of war.

• (3) Members of regular armed forces who profess allegiance to a Government or an authority not recognized by the Detaining Power.

• (4) Persons who accompany the armed forces without actually being members thereof, such as civilian members of military aircraft crews, war correspondents, supply contractors, members of labour units or of services responsible for the welfare of the armed forces, provided that they have received authorization from the armed forces which they accompany.

• (5) Members of crews, including masters, pilots and apprentices, of the merchant marine and the crews of civil aircraft of the Parties to the conflict, who do not benefit by more favourable treatment under any other provisions of international law.

• (6) Inhabitants of a non-occupied territory who, on the approach of the enemy, spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units, provided they carry arms openly and respect the laws and customs of war.

TL;DR: The people we are killing are in fact illegal combatants according to the Geneva Convention, and so terrorist is the correct word.

Ninja Edit: Source

Ninja Edit2: Formatting for an easier read.