according to some.

According to others, the difference is more than semantic as a guerilla can operate with or against a government or an occupier (The French Resistance in WW II) -- some call them irregular forces to introduce more semanticism or romanticism -- whereas an insurgent operates only against the government in the nation of interest (The OAS in France and Algeria during the Algerian Insurrection) generally with the object of regime change. Were that not so, he or she would not be insurging.

Insurgents and guerillas can use terrorism as a tactic in which case they become insurgents and guerillas who use terrorism as a tactic and may logically be called terrorists by some though they still remain insurgents and guerillas for all practical and (I am not a Lawyer) I suspect legal purposes.

Terrorists, OTOH use terror (i.e the threat of physical harm or the actual performance of acts that cause such harm) to coerce a group or nation (can be either) to do or not do certain things. Terrorists may or may not be insurgents or guerillas. If they are NOT sanctioned by a nation state, they are also likely to be called criminals as well. Unless Reuter's is reporting it -- then they become freedom fighters or something.