The FBI failed to understand the psychology of Koresh and the Branch Davidians. They treated them as criminals who used religion as a cover for perverted practices. They accused Koresh of everything from polygamy to child abuse. In their eyes, he was fickle and unbalanced, a maniac using his position to satisfy his lusts for power, sex, and drugs. They didn't take Koresh seriously as a religious leader, or student and expounder of the Bible.
Most criminals are non-ideological cowards.
People who operate from religious conviction are anything but cowards and are very ideological. You don't treat the two the same. Being of weak character, criminals succumb to pressure more readily, the better to save themselves. The more pressure you put on those who operate from principle, the stronger the group cohesion, the more determined the resolve, and the stiffer the resistance. The Branch Davidians are psychologically similar to the early Christians who preferred death at the hands of wild beasts and gladiators to the renunciation of their faith, and the 1st century Jewish zealots who committed suicide at Masada rather than surrender to the Romans.