Learn Italian: L'uomo è simpatico, ma il ragazzo è cattivo.

Just like "but" in English, ma connects two contrasting ideas. In Italian, you generally put a comma before ma when it connects two complete mini-sentences (clauses) that each have their own subject and verb, just like l'uomo è simpatico and il ragazzo è cattivo.

Does simpatico mean "sympathetic"?

No, this is a classic "false friend"! Simpatico means "nice", "pleasant", or "likable" when talking about a person's personality. If you want to say someone is "sympathetic" (understanding of someone's feelings), you would use a different word, like comprensivo.

Is cattivo the exact opposite of simpatico in Italian?

They contrast well in this sentence, but they aren't strict opposites. The direct opposite of simpatico (nice/likable) is antipatico (unpleasant). Cattivo means "bad" or "mean", and its exact opposite is buono (good).

Why do we use l' for uomo but il for ragazzo?