"That serious man is very bad."
Translation:Người đàn ông nghiêm túc đó rất tệ.
How did you translate the rest of the sentence? Maybe it's an error cause you've made a mistake during the translation before the "đó" part? That's when I'm wrong. When I've looked it up in the solution I've always had an error beforehand. By the way, I have the some solution suggested as you, but I've translated it wrong with "này". ;p