This dream is an emotional restoration dream. Your brother dreams about this because this is what he experiences in real life. Because of the drugs, he is a prisoner, from there the chains and the dungeon in the dream. The mine cart and the “rolled on” tracks also speak of him on a path that he cannot divert from. The family he protects is probably people close to him who are in the same situation as him. I do not think this has anything to do with his natural family unless he is blackmailed in real life. He is in bondage and he needs deliverance and counseling.
He is “forced” to do bad things by demons who are very active in his life. Because of the addiction, he also does things that bother him. The bottom line is, he does bad things. Not nearly as bad as it is depicted in the dream. He does not really slaughter people. However, it is almost as bad, because he does slaughter people in the spiritual realm and he does enjoy it, as is depicted in the dream.
Many people will never kill and slaughter people in real life, but, spiritually, they do not have a problem with that. They will gossip, and they will hurt people in their hearts. They will curse people and they will secretly wish that bad things happen to them. They will even enjoy such thoughts. The Bible says that this is already murder.
The red blood speaks of many conflicts and the black speaks of a lot of demonic activities, depression, and even hopelessness.
This type of dream is of course a nightmare (emotional restoration dream) but it also shows what is really going on in our lives. It looks and sounds a lot more serious than it really is, but, on the other hand, it sometimes is serious.
I think your brother is in a bad place and there is a lot of guilt and remorse involved. Watching the wrong movies is a bad choice and although it triggered the dream, the symbols in the dream do indicate what is in his heart (soul). He admitted that he had such dreams in the past so the bad movie is just the trigger but not what this dream or the others are all about.
When we look at a person and when we judge a person, they may not look bad to us, they can even appear to be good people, but, that is not how God sees them. God sees the secret sins of the heart. The Pharisees in New Testament times were respected and regarded as good people but Jesus knew who they were in their hearts. Just read Matthew 23. I think the Jews could not believe their ears when they heard Jesus condemning them.