Is it normal to imagine "very bad things" happening every now and then?

It is normal. to a degree; although, some people who suffer from Obsesive Compulsive Disorder are plagued by negative, uncontrollable thoughts and they often perform little rituals like washing hands many times a day, touching something a certain number of times etc, as they feel they HAVE to or the bad thoughts will come true.. something you should consider checking out. It is very common and like I said, everyone has their little quirks to some degree.