What happens if you spare Whoreson?

If you spare him, he becomes a hobo and you see him getting attacked by some kids. If you kill him, Dudu takes over his identity and turns his business legit.

Is Dudu impersonating Whoreson?

Dudu disguised himself as his one-time adversary, Whoreson Junior, and took over his business, announcing it was going clean and investing all of its ill-gotten wealth in a legal (and very profitable) overseas trading company. In a way, you could say he was living the “Novigrad dream.”