If a murderer is showing signs of being the murderer in a game (i.e; Showing a knife, sprinting, or holding a grey dye) and a bystander has a gun and decides to not shot and attempt to kill the murderer but instead tags along with the murderer and does nothing to end the game, then those players are subject to a warn and even possibly a kick. The same applies to bystanders who are obviously helping the murderer by killing other bystanders.
Teaming by definition, is to come together to achieve a common goal. So after discussing this with the rest of staff we have come to the conclusion that, leaving a bystander for last, is not teaming so therefore it is not punishable.
As for ghosting, it is frowned upon and we would like for players to respect the game and not do so, but it is not punishable.
FYI: The trick to reporting possible teaming in murder is to record the match. Unless you can obtain a large amount of screenshots that show undeniable teaming.