There is a big difference between cults and mainstream religion.
First of all not all cults are even religious. Cults can be based upon politics, martial arts, meditation, yoga, multi-level marketing schemes, therapy, large group awareness training, UFOs, various philosophies, etc.
The most salient single feature of a destructive cult is a living leader that defines the group. He or she is the focus of the group and the locus of power. That authority figure has no meaningful accountability.
The second feature is what has been called "cult brainwashing". Members are put through a process within the group that essentially shuts down their critical thinking and engenders dependency upon the group/leader to think for them. Whatever the leader says is right is right and whatever the leader says is wrong is wrong.
Finally for the group to be a destructive cult it must engage in destructive behavior, which is mandated by the leader and systemically produces damage concerning the membership. This varies by degree from group to group. In some groups the damage done is psychological and emotional in others this can be more extreme, e.g. sexual abuse, medical neglect or violence.
Psychiatrist Robert Jay Lifton's paper "Cult Formation" explains this in more detail. Lifton also explains "thought reform", which has been popularly labeled as "brainwashing".
Cults can become religions over a period of time, but they are not like mainstream religions in the beginning. Most cults fade away after the death of the founding leader. Only a few have evolved after that point to become a religion or denomination.
This can be seen as somewhat analogous to Eric Hoffer's observations about the evolution of revolutionary movements in his seminal book "True Believer". Hoffer saw such movements as moving through stages of development and said some can potentially become part of the status quo after they evolve through the most turbulent active phase.