We cannot assume that the Zurich gunman acted alone. Unfortunately, we live in an era where things like that are all too common. Individuals are becoming radicalized and compelled to act in horrifying ways. This results in incidents like what happened in Zurich. We should always be aware when these things happen and prepare ourselves accordingly.
Though this man should not be labeled as a terrorist without proper evidence to support the accusation, the growing number of terrorist-related activities worldwide do make it seem likely. By all rights, an act of violence like this can be considered an act of terror. Whether he was part of a larger organization remains to be seen, however.
According to the Zurich police, the man was not a radical Islamist, so it's unlikely that he was a terrorist. Moreover, the attack was carried in a mosque, which may imply the case is discrete. The man had a juvenile record of bike theft and assault, and he later shot himself after the incident.
Police found no known terrorist affiliations for the Zurich gunman, and determined that he acted alone in shooting three people and stabbing one other person. Reports indicate that he acted alone, after recently quitting his job. Police found occult materials in his apartment, but nothing that might indicate any motives for the killings.