Answer:
Dangerous secret
Step-by-step explanation:
It is given that Carrie and Craig have been married for two years. But quite often the couple ended up in fighting that results in violent and it becomes physical.
Doug, who lives in the apartment below them and is a cousin of Carrie, knows about Carrie and Craig's quarrel and their violent fighting. But he keeps it secret and does not informs anybody because he is afraid of the law.
But Doug's this act is dangerous and as a good citizen and being a family member of Carrie and Crag should informed other or the law. If he keeps quite their fighting may turn up to be more violent sometime and it will cause more harm to Carrie and Craig later on, which may lead to serious injuries and may be loss of life.
Thus the secret that Doug keeps is a dangerous secret and it will cause more harm to Carrie and Craig.
Thus the answer is dangerous secret.