I propose an alternative explanation. Some people are just born psychopaths; they love to hurt other people.

Whatever nice cause you start, if it gains just a little power, sooner or later one of them will notice it and decide they like it. Then they will try to join it and optimize it for their own purposes. You will recognize that this happened when people around you start repeating memes that hurting other people is actually good for your cause. Now, in such environment people most skilled in hurting others can quickly rise to the top.

(Actually, both our explanations can be true at the same time. Maybe any movement that doesn't open its doors to psychopaths it doomed in the long term, because other people simply don't have enough power to change the society.)