L'incidente ha ucciso due persone.

Because il becomes l' before a singular noun that starts with a vowel.

• il incidente → l'incidente
• il amico → l'amico

This is called elision. The apostrophe shows that the vowel in il has been dropped.

Ha ucciso is the passato prossimo of uccidere (to kill).

It is made with:

• ha = has / auxiliary verb from avere
• ucciso = past participle of uccidere

So literally, it is something like has killed, although in English we often translate it simply as killed depending on context.

Because uccidere normally takes avere as its auxiliary in compound tenses.

So:

• ha ucciso = killed / has killed

Not:

• è ucciso for this meaning

Also, uccidere is a transitive verb: it takes a direct object, here due persone. Transitive verbs usually take avere in Italian.

Italian, like English, can use an event or thing as the subject of a verb of causing harm.

So L'incidente ha ucciso due persone is structurally very similar to English The accident killed two people.

That said, Italian often also uses other ways to express this idea, especially in news style, such as:

• Nell'incidente sono morte due persone = Two people died in the accident
• L'incidente è costato la vita a due persone = The accident cost two people their lives

Your sentence is still perfectly understandable and grammatical.

Because the singular noun is persona, and its regular plural is persone.

• una persona
• due persone

So personi is not correct.

Because with a number, Italian normally does not use an article before the noun.

So you say:

• due persone = two people
• tre libri = three books

You would not normally say le due persone unless you mean the two people, referring to specific people already known in context.

Because grammatical gender in Italian belongs to the noun, not necessarily to the real-life sex of the people involved.

Persona is grammatically feminine, even if it refers to a man.

So you say:

• una persona
• due persone
• questa persona

Even if one of those people is male, the noun itself stays feminine.

You could, but the tone changes.

• uccidere / ha ucciso is the more standard and neutral choice
• ammazzare / ha ammazzato is more colloquial and can sound rougher or less formal

For something like a report, article, or neutral statement, ha ucciso is usually the better choice.

The basic and most neutral order is:

• Subject + verb + object
• L'incidente ha ucciso due persone

You can move things around in Italian, but that usually adds emphasis or changes the tone.

For example:

• Due persone le ha uccise l'incidente is possible in a marked, emphatic context, but it is not the normal basic sentence.

So the given order is the standard one.

Since persone is feminine plural, the direct object pronoun is le.

So:

• L'incidente ha ucciso due persone.
• L'incidente le ha uccise. = The accident killed them.

Notice two things:

1. le goes before the auxiliary
2. the past participle often agrees with a preceding direct object pronoun, so ucciso becomes uccise

It can correspond to either one, depending on context.

Italian passato prossimo often covers both:

• a simple past meaning
• a present-perfect-like meaning

So ha ucciso could be translated as:

• killed
• has killed

In many everyday contexts, especially when telling what happened in an accident, natural English would often be The accident killed two people.

A helpful approximation is oo-CHEE-zo.

A few key points:

• u = like oo
• cci before i sounds like chee
• s in -so here sounds like a voiced z in standard pronunciation for many speakers, so -zo

So:

• ucciso ≈ oo-CHEE-zo

And the stress falls on -ci-:

• uc-CI-so