Did they use gas chambers to kill people just anytime or a certain time?
Of course, they did it at the convenience of the NAZI officers. They didn't want to endure the late hours.
They did that so the family of the people could never be buried or actually have a place to go back to, to feel better.
Do ghost haunt nasty people?
Not just nasty people...Sometimes spirits or "ghosts" haunt people because they have a certain connection, i.e. the home, a certain person, a certain object, etc. Sometimes the connection is never apparent, so it seems as though the spirit came from nowhere. But no...not just nasty people...or people with nasty dispositions.