If the intent of a person with a gun is to kill as many people as possible with it...

[Xipe Totec]

8. Definition of Terrorism

There is neither an academic nor an accurate legal consensus regarding the definition of terrorism. Various legal systems and government agencies use different definitions. Moreover, governments have been reluctant to formulate an agreed upon, legally binding definition. These difficulties arise from the fact that the term is politically and emotionally charged.

Angus Martyn in a briefing paper for the Australian Parliament has stated that "The international community has never succeeded in developing an accepted comprehensive definition of terrorism. During the 1970s and 1980s, the United Nations attempts to define the term foundered mainly due to differences of opinion between various members about the use of violence in the context of conflicts over national liberation and self-determination." These divergences have made it impossible to conclude a Comprehensive Convention on International Terrorism that incorporates a single, all-encompassing, legally binding, criminal law definition of terrorism.

https://en.wikipedia.org/wiki/Definitions_of_terrorism

So much for your planetary consensus.