Learn Dutch: Wie pleegt zo'n misdaad? - Who commits such a crime?

Zo'n (a contraction of zo een) is used before singular countable nouns like misdaad (crime). If the noun were plural (crimes) or uncountable (like money or weather), you would use zulke instead, for example: zulke misdaden (such crimes).

Does plegen always mean "to commit"?

Usually, yes. It is the standard verb for committing a crime (een misdaad plegen) or a murder (een moord plegen). However, you might also see it used formally to mean "to make" (een telefoontje plegen = to make a phone call) or "to be accustomed to" doing something (hij pleegt te komen = he is accustomed to coming).

Why is the verb plegen conjugated as pleegt when we don't know if multiple people committed the crime?

The question word wie (who) is grammatically treated as a third-person singular subject in Dutch, just like in English. It always takes the singular conjugation (stem + t), even if you strongly suspect a group of people did it.

How would the predicative resumptive het from this lesson be used in a conversation about this?