Learn French: Il a attaqué un homme, il est coupable !

You would say Il n'a attaqué personne. Remember that personne (nobody) goes after the past participle (attaqué). If you wanted to say 'He attacked nothing', rien (nothing) would go between the helping verb and the past participle: Il n'a rien attaqué.

Is there a liaison when pronouncing un homme?

Yes! Because homme starts with a silent H, you pronounce the N from un as if it is the first letter of the next word. It blends together to sound like 'un-nomme'.

Why doesn't the past participle attaqué change to match the subject?

Because the helping verb is avoir (a). When using avoir in the passé composé, the past participle does not agree with the subject. It only changes if a direct object comes before the verb. Since the object here (un homme) comes after the verb, attaqué stays in its default form.

The word coupable looks like the English word 'culpable'. Are they related?

Yes, they share the same origin! While 'culpable' is a formal word in English meaning deserving blame, coupable is the standard, everyday French word for 'guilty'.