Non minacciare nessuno.

Because this is a negative command addressed to tu. In Italian, the negative tu imperative normally uses:

non + infinitive

So:

• Minaccia! = positive tu command
• Non minacciare! = negative tu command

This is a very common pattern:

• Parla! → Non parlare!
• Vai! → Non andare!
• Mangia! → Non mangiare!

It is talking to one person, informally: tu.

So Non minacciare nessuno is the kind of thing you would say to one person you address as tu.

If you were speaking to more than one person, you would say:

• Non minacciate nessuno.

To one person formally, using Lei, you would say:

• Non minacci nessuno.

So the three useful versions are:

• tu: Non minacciare nessuno.
• Lei: Non minacci nessuno.
• voi: Non minacciate nessuno.

Because Italian normally uses negative concord.

When words like nessuno, niente, mai, and similar negatives come after the verb, they are usually used together with non.

So:

• Non minacciare nessuno
• Non vedo nessuno
• Non dico niente
• Non vado mai

This is completely normal in Italian and does not sound like incorrect double negation.

In this sentence, the most natural English translation is usually anybody / anyone:

• Don’t threaten anyone.

But the Italian word is still nessuno.

Depending on context, nessuno can correspond to English no one / nobody / anyone. In negative sentences, English often prefers anyone, while Italian still uses nessuno.

No. Minacciare normally takes a direct object, so you say:

• minacciare qualcuno
• minacciare nessuno

not:

• minacciare a qualcuno

So in Non minacciare nessuno, nessuno is the direct object of minacciare.

Because Italian usually leaves out subject pronouns when they are clear from the verb form or the situation.

In commands, this is especially normal. So:

• Non minacciare nessuno.

is more natural than:

• Tu non minacciare nessuno.

You can add tu for emphasis or contrast, but it sounds more marked.

It is a regular -are verb: minacciare = to threaten.

Some useful forms:

• io minaccio
• tu minacci
• lui/lei minaccia
• noi minacciamo
• voi minacciate
• loro minacciano

The spelling with -ci- helps keep the ch sound, as in English chair.

Yes.

For example, if you mean him, you could say:

• Non minacciarlo!
• Non lo minacciare!

With negative tu commands, object pronouns can often either:

• attach to the infinitive, or
• come before it

So both of the examples above are possible.

For other command forms, the pronoun normally goes before the verb:

• Non lo minacci. = formal
• Non lo minacciate. = plural

In this specific command, the normal order is:

• Non minacciare nessuno.

Putting nessuno before the verb would sound unusual here.

More generally, nessuno can appear before a verb in other sentence types, especially as the subject:

• Nessuno ti minaccia. = Nobody is threatening you.

But for this command, the standard pattern is non + verb + nessuno.

Approximately:

non mi-nac-cià-re nes-sù-no

A few helpful points:

• cci in minacciare sounds like ch in chair
• the stress is on -cià- in minacciare
• the stress is on -sù- in nessuno

So the strongest beats are:

• minacCIÀre
• nesSÙno