I think the habit for using the words "terrorist" and "terrorism" to refer only to illegal non-government organisations, and to any activities carried out by such organisations, ought to be resisted.

"Terrorism" means violence against civilian targets, used as a way of trying to bring about political change, or prevent it. Anyone who does that is engaged in terrorist activities, regardless of whether the organisation they are working for has legal status ot not.

And activities carried out by illegal organisations which do not fall into that category - for example attacking military units - should not be referred to as "terrorism". That isn't to justify these. But words should be used accurately.