I assume when you say "cult" you mean destructive cults. The word cult is being removed from the vocabulary and being replaced with "new religions" in an attempt to legitimize some religions that aren't Christian or don't conform to Biblical Christianity, and who aren't destructive.
My definition of a cult is any group that isolates people, is abusive or controlling, or who has a charismatic leader who believes he or she has discovered the ultimate hidden truth of scripture.
I was involved for a short time with a group called Twelve Tribes. They were a Jewish roots group. They didn't isolate anyone really, at least not geographically; but they try to get you to leave everything and join them. Of course, they had the only true revelation and interpretation of scripture and they try to brainwash you. I'd call them destructive because I became very confused about what I believed for a long time after leaving.
People's Temple was definitely destructive. Unfortunately, it wasn't a mass suicide, it was murder plain and simple. Most of those people didn't want to kill themselves but Jones brainwashed them, and armed guards stood by and forced people to do it. He compared Jonestown to Masada where everyone committed suicide rather than being taken captive by the Romans. He had a persecution complex and believed Jonestown should make a stand similar to that of Masada.