Learn Italian: Da poliziotto, lui ha visto molte cose brutte. - As a policeman, he saw many bad things.

In Italian, the preposition da is used before a profession, role, or stage of life to mean 'in the capacity of' or 'as a'. You will also see this in common phrases like da bambino (as a child) or da giovane (as a young person).

Shouldn't it be da un poliziotto since the English translation says 'As a policeman'?

No, when da is used to indicate a role or profession, you drop the article completely. Adding the article (da un poliziotto) would change the meaning to 'by a policeman' or 'from a policeman'.

Is the pronoun lui necessary here?

It is not strictly necessary, since ha visto already tells us the subject is he, she, or it. However, keeping lui adds a little emphasis. It highlights that he specifically saw these things, perhaps in contrast to an ordinary civilian.

Why does molte come before cose but brutte comes after?

Quantifiers—words that tell you how much or how many, like molte (many) or poche (few)—always go before the noun. Descriptive adjectives, like brutte (bad or ugly), usually go after the noun they describe.

Why use the passato prossimo () instead of the imperfect ()?