A religious sect is born of disagreement and a cult is born of misplaced worship. Neither is conducive to the growth of souls or the betterment of society. Both have resulted in untold suffering over the millennia.
Sects are splinter groups from a parent religion. They often form as a result of schism. Others form as an orderly result of a process controlled by the parent religion. Sects weaken the whole. Sects dissipate the vigor and resilience of the community.
A cult, on the other hand, may parade around as a religion or a sect but it operates in the dark. Its leaders may be charismatic and intelligent but their beliefs and practices are usually radical and unpopular. They tend to be secretive and are often paranoid about outsiders. Cults are always the result of negative forces working against the best interests of the community.
The believers in a sect or a cult can be quite advanced in their thinking and beliefs, or wayward and depraved. They can be very fanatical and intolerant. As such, they do not adhere to all the teachings and principles enunciated by the founder(s) of their religion, sect, or group and they go off on their own bringing disgruntled believers with them and seeking out new ones, openly or covertly.