Power? Control? Indoctrination and lack of critical thinking skills to escape it? Some kind of latent fear where they think if they don't come out on top someone will, treating life as a zero sum game? There's lots of potential reasons for people to act this way but it probably depends on the individual. I'm not comfortable painting with a super wide brush here because its dehumanizing, and these people are human. Reprehensible? Perhaps, but still human.

We can wax philosophic about why anyone does the things they do, but in the end it doesn't matter why they do what they do, it matters what they do and what actions we can take to try and improve our own lives and the lives of the people we care about.