That's a pretty generic thing to say. I mean what exactly is a "bad dog". he probably just needs to be trained.

Maybe he's stupid lol. I had a labrador retriever and it was house trained in like 2 days.

How much time is left alone and is he an only dog?

I don't think any dog is really "bad", but some of them have behavior issues that need to be sorted out.
We used to have a dog like that named Sashee actually, she was this adorable pit bull/rottweiler mix but she had quite a temper.
The neighbors got pissed by her never ending barking sessions and we couldn't find a way to fix her temper so we wound up giving her to this really nice guy that owned her mother.
She seemed happier with him too, I guess because he knew how to keep her calm.