Why do Russian troops make people vanish in Ukraine?

FSB armored vehicle (Redditt)

Vanishing is the thing that causes fear. Fear is the thing that makes people cooperate. And the purpose of fear is to make people remain silent about things that can harm somebody. The political police or secret police use this kind of method where a person just vanishes. There is no trace or evidence of their fate. 

The idea of this kind of action is similar to all other secret police psychological operations. Many political or secret police organizations like Gestapo, NKVD, and DINA (Chilean security police) maximize their fear by taking their victims to arrest facilities at night time. Because those people who are making those arrests are coming at night time. That allows them to hide their identity. Because people cannot see the faces of men of the secret police, that makes those persons even more terrible. 

The purpose of making people vanish is to cause fear and uncertainty. If a person is dead people believe that there is nothing that can hurt those people. That can make those people's friends fight hard and tell something that some people don't want to tell others. 

But if a person has just vanished there is always a small hope. And the purpose of that brutality is to control those people who are left behind. The vanishing causes uncertainty about what is happening. And the fear about fate, what happened in the arresting place is horrible. The thing is that fear is the thing that the secret police normally use for controlling people. 

Those organizations are delivering information that anybody can be their informer. And if somebody talks something against the state. Others must make information to the secret police immediately. Because in the cases that the secret police hear about that agitation from some other source. That thing would cause, that also other people who "hired" that information will send to prison.