They were arguably the most repressive secret police regime in the world. They disappeared people and their families, and used torture, intimidation, and the biggest network of informants ever assembled (in some cities almost 20% of the population were informants!).

People collaborated because they had to, on top of the usual reasons for joining a military or intelligence service.

Probably the same as for ICE today.

Same list of things that compels people to become an asset for any intelligence agency, adjusted for the time in history.