>A terrorist is someone who uses violence and intimidation against civilians.

You forgot to add "in order to push some sort of cause". Someone who uses violence or intimidation for no other reason than they are just mentally deranged and want to harm others isn't technically a terrorist. To fall under that label the person must have some sort of agenda or motive besides just wanting to hurt people, and most often the motives are either religious or political in nature. If the definition of terrorist was nothing more than "someone who causes terror" then I'd agree with you, but that isn't an accurate description which is why not every shooter is labeled as such. The persons religious affiliation only matters if their religion was the motive behind the crime, and that applies to all religions, and also all political ideologies.