Your Phrase in English
If you want to directly render the Farsi saying in English, this might be a more understandable translation:
Going after the drunks would mean going after everyone in the city.
Going after the drunks would mean arresting the entire city.
- "Going after" stands alone better and more clearly suggests the pursuit of a criminal than "get." It is a phrasal verb that is not really related to the prepositional meaning of "after."
- There is also no need to say, in the passive voice, "if it is decided that," since "would" already implies that the subject (the verb phrase "going after the drunks") is a future hypothetical.
- "Arresting the entire city" will be understood colloquially to mean "arresting [everyone in] the entire city," and is an example of metonymy.
These suggested shorter phrases have more punch (effect, memorability, ease of understanding).
The following known sayings aren't perfect matches to your presented phrase, thus aren't direct substitutions, but they could work.
You should count the cost first.
Meaning: Make sure, before beginning any enterprise or project, that you can actually afford it, or you may end up paying more than you wish or can afford. Cost, here, can be non-monetary, such as time, effort, or any negative consequences.
In for a penny, in for a pound.
Meaning: This course of action will be hard to exit from later. "In" means "to be entered into at the cost of," similar to asking a people playing high-stakes card games how much money they have at risk, "how much are you in for?" There is a sense of becoming locked in to events or to an enterprise such that a small initial commitment or cost will likely balloon into a large one (or has already), with little chance to escape. Again, while expressed in monetary terms, the concept extends to any kind of cost or negative consequence. A penny is a small unit of money, and a pound is a large unit of money (see monetary symbol GBP).
An eye for an eye will make the whole world blind.
Note that this is only for special situations and isn't a general replacement for your phrase.
Meaning: It isn't wise to take revenge or give out exact retribution, since everyone makes mistakes, and the end result of such a policy (taken fully literally) would result in a lot of eye-gouging, causing many to be blind. Limited to when the proposed action (being likened to cutting out someone's eye) could be considered some kind of retaliation.