Learn Italian: L'egoismo è un difetto che distrugge l'amicizia. - Selfishness is a flaw that destroys friendship.

While -ismo is heavily used to form nouns for political, philosophical, or artistic movements (like fascismo or cubismo), it is also commonly used to describe attitudes, character traits, or behaviors. In egoismo (selfishness), the suffix is added to the Latin word ego (I) to describe a self-centered attitude rather than a formal belief system.

If egoismo is the trait, how do I use the -ista suffix to talk about a selfish person?

You would use the word egoista. As a quick refresher on a tricky rule for -ista nouns: in the singular, they end in -a for both masculine and feminine people (e.g., lui è un egoista, lei è un'egoista). However, their plurals change normally according to gender: gli egoisti (masculine/mixed) and le egoiste (feminine).

Why are there definite articles before egoismo and amicizia (l'egoismo, l'amicizia) when we don't say "the selfishness" or "the friendship" in English?

This is a classic difference between the two languages that is easy to forget. In Italian, definite articles are almost always required before abstract nouns when talking about them in a broad, general sense. English omits the article for general concepts like "selfishness" and "friendship", but Italian strictly requires them.