Doesn't 被 mean "by"?
It suggests the passive mode as "be done" in English. So "被" + verb = be done in English.

About the "by", just as English, when using the passive mode, not all the sentence has a "by" in it.  

Example with "by":

active: 

A lion ate the monkey.(狮子吃了那个猴子)

passive:

The monkey was eaten by a lion.(那个猴子*被*狮子吃了)

Example without "by":

He was killed,(他*被杀*了) but we don't know the murderer yet.

Here there is no "by" because we all know he was killed by someone we don't know yet. So no need to add "by the unknown murderer" here. Same as the Chinese version.