In a sense every religion or form of worship (fandom included) is a cult. Cult gets its bad name from being associated with those that go against the grain, not to mention the mass suicides and stuff.
If you are part of a cult you don’t call it a cult. It is your form of worship. Someone on the outside calls it a cult because they don’t believe what you do. To the Sith, the Jedi are a cult. To the Jedi, the Sith are the cult. It really comes down to philosophical differences. But generally when some massive group of worshippers kill themselves and let’s say they are Muslim or Christian or whatever. The “mainstream” Muslim or Christians will label that group a cult of their faith. ‘They did not follow the “true path”’. When in reality they did not follow common sense. Suicide is never an answer. A martyr dies alone and at the hands of someone else. A martyr does not kill others in the process.
Unfortunately basic human truths that have been found by religions have been distorted and abused by extremists. It’s sad but very human. It’s relevant now and thousands of years ago.