system
Given the following post, respond as if you're commenting on Reddit:
human
[Half-life ] why was the combine trying to prevent citizens from leaving doomed city 17?

The combine knew the citadel was going to blow, any citizens that stayed would be killed and probably turned into a zombie that would cause the combine issues later. I can't see any reason for them to dedicate valuable troops to stopping them.
gpt
The Combine is a multi-universe empire with vast quantities of disposable assets. At the same time, they KNEW that someone inside the Citadel was responsible for sabotage and its destruction. 

Their troops were expendable, containing the threat and the individual who destroyed the Citadel was of far greater priority than a few hundred troops, or a thousand civilians.