So what happened in New Zealand would be labeled terrorism or "mentally unstable" because the killer was not Muslim?
"Terrorism is a contested term, with no set definition for the concept or broad agreement among academic experts on its usage."
Some define it as violence or even the threat of violence in service of a political aim. If there is no political declaration of intent, it's not considered terrorism. Others may consider any violence against civilians a terrorist act.
This could be why it's classified or labeled differently.
I'd hate to think it's because the perpetrator wasn't Muslim.