I'm assuming you're privileged enough to have never been an enlisted soldier in a military? This isn't hordes of men clamouring to "oppress" someone. Often people are conscripted or come from such a poor background that enlistment helps their life significantly.

After the first 6 months in the military, you'll have been broken enough to have lost a lot of sense of individuality and are willing to just blindly follow orders. After your initial training, you're likely dumped into an established unit where you are at the very bottom of the totem pole. Until about 20 years ago in western militaries, you'd experience physical violence on the first day of your first posting into a "real" unit just to show you your place. I'm assuming this still happens in China. If there are soldiers in those trucks it is very likely they haven't been told a single thing about their mission.

It's very easy to live your comfortable life and talk about how evil the individual soldiers are but in reality, the troops holding the rifles on the ground are simply a tool of those higher.