Can we change sentence having no subject into Passive voice? E.g.
a) 2 person were killed.
b) A person was electrocuted.
Is possible to identify if sentence is Passive voice or informative or state. Without reading further context?
a) 2 person were killed in accident.
b) A person was electrocuted in yesterdays mishap.
I think, perhaps, you are confusing two unrelated grammar rules. The grammatical rule for passive voice is simple, and is what JavaLatte said it is.
However, rules for stative and dynamic verbs are (partially) what determine whether passive voice is talking about state or not.
A transitive verb that is always stative can be in active or passive voice, but when it is in passive voice, it is always talking about state:
Active: Everyone liked her.
Passive: She was liked.
Her state was "liked," because the verb "to like" is always stative. This can't possibly be referring to any kind of dynamic past action.
When used in the passive voice, a verb that is dynamic can usually only refer to an action that happened (not state). Both of the verbs you chose in your question are examples of this. However, this isn't a hard and fast rule, some dynamic verbs (such as break) do have past participles that can refer to state.
In short, if it follows the grammatical construct for passive voice, then it is passive voice. Whether or not it refers to state depends entirely on whether or not the past participle always, never, or sometimes refers to state.
If you have a sentence whose construction is
[noun] [be-verb] [past participle]
it must be passive voice. Your two sentences are both passive voice: no context is required to determine this.
There is some possibility for confusion is a word can be either a past participle or an adjective, for example broken:
The window was broken on Thursday.
If broken is a past participle, this is passive voice: we are talking about somebody breaking the window on Thursday.
If broken is an adjective, the state of the window on Thursday was broken: it actually broke some time before that.
In such situations, context is required to identify the correct meaning.
My car was stolen last night.
This phone was made in Korea
If you know who the agent is, and it is important for the sentence, you would normally use it as the subject in an active voice sentence:
That man stole my car!
You only use passive voice and specify the agent if the object/patient is the most important thing in the sentence, and the agent is of some lesser relevance to the story:
My husband was attacked by two youths last night.
He killed two people (active)
"He" is the subject, "two people" are the object of the verb "kill". In your sentence a) the agent, the subject that was in the active voice, is missing but it is understood by the speaker.
a) 2 people were killed. (passive)
"2 people" are the subject in this sentence, in grammar the subject in passive clauses is referred to as the patient or the recipient, which means the thing or person(s) that received the action. WE, the reader, do not know who or what killed the two people. They might have been murdered by someone or they might have been killed in a road accident. WE do not know unless more context is provided but, usually, the speaker does know.
Note that I changed the singular noun "person" to its more conventional plural form, "people". ACTIVE
The singular noun, person, was changed to the plural form (by me) PASSIVE
See Wikipedia's article on the passive voice on how to identify the English passive
