From Book 6, Chapter "A Sluggish Memory"
`As he moved up the school, he gathered about him a group of dedicated friends. I call them that, for want of a better term, although as I have already indicated, Riddle undoubtedly felt no affection for any of them. This group had a kind a dark glamour within the castle. The were a motley collection; a mixture of the weak seeking protection,the ambitious seeking some shared glory, and the thuggish, gravitating towards a leader who could show them more refined forms of cruelty. In other words, they were the forerunners of the Death Eaters, and indeed some of them became the first Death Eaters after leaving Hogwarts.ยด
I think Jo shows some of the reasons for joining Voldemort in this quote. Once they had joined, there was no way out. Some of them stay out of fear (Wormtail).some are simply sycophants and some are cruel, insane persons themselves (Bellatrix Lestrange).
