Learn Italian: L'assassino non ha nessuno scrupolo e nessun pretesto. - The murderer has no scruples and no pretext.

This follows the exact same phonetic rule as the indefinite articles un and uno. The adjective nessuno drops its final -o to become nessun before vowels and most consonants, like the p in pretesto. However, it must remain nessuno before an "impure s" (an s followed by another consonant, like the scr- in scrupolo), as well as before z, gn, ps, and pn.

The English translation says "no scruples" (plural), so why does the Italian use the singular scrupolo?

In Italian, when nessuno is used as an adjective meaning "no" or "not any," it is almost strictly used in the singular form. While English speakers naturally say "he has no scruples," Italians phrase it as "he has not a single scruple" (non ha nessuno scrupolo).

Why is there a non in front of ha if the sentence already uses the negative word nessuno?

Italian requires a double negative in this structure. Whenever a negative word like nessuno, niente (nothing), or mai (never) comes after the verb, you must place non before the verb. If nessuno started the sentence as the subject (for example, Nessuno parla meaning "Nobody speaks"), you would drop the non.