According to Jonathan D. Spence, Hong Xiuquan’s belief system started out as ordinary. Early on, Hong adopted the prevailing ideology of the period. He had no aims to take on China's established rulers. He even took the Chinese imperial examinations.
After his belief in the status quo failed to earn him any success (according to Spence, Hong failed the examinations four times), Hong dabbled in Christianity. Soon, he’d use a dream he had to support his argument that he was Jesus’s son.
Now, Hong’s belief system revolved around him. He believed that the High Heaven appointed him to “slay the demon devils.” As Spence points out, those demon devils were the members and supporters of the Qing dynasty. Hong presented a magical, intoxicating vision of himself and China. His mystical charisma garnered him dedicated followers, who would, as your question notes, fight and kill for him.
At first, Christian missionaries thought Hong might be a positive sign that Christianity was spreading throughout China. However, as Hong’s political program took hold, it became apparent that Hong was using Christianity as a pretext to turn himself into an all-powerful ruler.
Hong professed a communal ideology. Spence includes a quote from Hong where he declares that “every place shall have equal shares and every one be clothed and fed.” However, in practice, Hong’s regime was deadly and repressive. It was also hypocritical. He outlawed sex yet kept a harem for himself.