1-Translate into Arabic "Habil was killed by Qabil" 2- What are the uses of the passive in English?

الترجمة هي: قابيل قتل هابيل.

Not like Arabic, the English language tends to use the passive voice more; it is sometimes called the "language of passivity".

Anyhow the passive is used in English commonly when the real doer of the action, which we call it the "agent", is unknown or not mentioned; no need to mention that the grammatical subject is any noun or pronoun that begins a sentence. But it is not restricted to that only, as we can see in the sentence in the question: "Habil was killed by Qabil"; here both the gramatical subject and the agent are mentioned.

Arabic language does not tend to use passive forms usually. If we translate the above exemplary sentence into Arabic, we usually put it in the active form, as Arabic language usually tends to work.  

قتل قابيل هابيل (فعل-فاعل-مفعول

Passive voice is used when :The subject (who did the action) is unknown

Example :

The president was murdered yesterday (we don't know who has killed him)

or 

It is obvious to the listener or the reader who the subject is because it is the person who usually does it .

Example: 

The cleaner cleaned the room  

In the case you mentioned (Habil was killed by Qabil) it is a passive  case  for the active sentence (Qabil Killed Habil) 

Let's see how was this formed : 

Qabil   Killed  Habil (active )

Subject / verb / object      

It was formed to be

Object + verb +by + Subject

Habil was Killed By Qabil

Note ::::

Some verbs can't be changed to active voice Example :

Ahmad slept well.

هابيل تم قتله بواسطة قابيل.

 Passive voice is used when the action is more important  than the one who did it

قتل هابيلَ قابيلُ

أو 

هابيلُ قتله قابيل

In Arabic, we do not use the passive unless the doer is totally unknown. Instead, we topicalize (move to an earlier or front position) the object (receiver of action) as shown above.

We use the passive in English to when the doer is unknown, logic, not important, or concealed.

الترجمة: لقد تم قتل هابيل من قبل قابيل

As has been said in the answers above, the passive voice is used when the subject is unknown or when the action is more important than the subject. 

قتل قابيل أخاه هابيل (فعل وفاعل ومفعول)

Actually such a sentence should not be in passive voice because everybody knows that Qabil killed Habil.