Question

She looked at me as if I were a thief so why they write as if I were a thief

Answer 1

because they want to say that he was not thief but she asume him a thief.

Answer 2

Actually the sentence is imagination.
You are not thief I'm real,but she assumed that u are theif.
So,they used the word as if.
the word as if is used for giving stress to the sentence.

Hope this helps you quickly