'As' is used to mean a real thing.

John acted as an old man. ( John is an old man)

John acted like an old man. (John is not an old man, 'like' describes the way he acted)

She works as a nurse. (She really is a nurse)

She treats her mother like a nurse. ( She is not a real nurse)

The man works as a slave. (He is a slave)

The man works like a slave. ( He works so much that he resembles a slave)

As your brother, I must warn you the danger of smoking. ( I am your brother)

Like your brother, I must warn you the danger of smoking. (I am not your brother, but he and I have the same advice)

After a negative clause, a comparison with 'as' or 'like' usually refers only to the positive part of what comes before.

I don't smoke, like John. ( John smokes)

I am not rich, like Tim. ( Tim is rich)

Before a negative clause, the comparison refers to the whole clause.

Like John, Tim doesn't smoke. ( John does not smoke)

Like Jim, John is not rich. (Jim is not rich)

(Reference: Practical English Usage - Michael Swan - Oxford)