In Downgrade, you have no set teams and it is a complete free for all. Anyone can attack anyone, that means even multiple people can attack one player at once because it is a free for all of pvp, and your goal is to kill the other players.
In Murder, you have two teams, the Murderer and the Bystanders. The goal of the Murderer is to kill all bystanders and the gold of the bystanders is to kill the murderer. They have separate set goals and separate teams, so if the murder teams up with a bystander it is considered illegal teaming.
If you have any other questions about this feel free to PM me with any other concerns!