Yeah the fact that Caboose was an inno following a detective completely renders that argument null.
Here is the way I see it:
•Innocent A tells Innocent B to stop following him and gives him the proper amount of warnings. At that point Inno A has the complete right to defend themselves against Inno B. Inno B also has the right to fire back AFTER being fired upon/hit, as this too constitutes self defense. It is also possible that one of them is a traitor, hence why both have the right to fire upon each other.
•I would consider it RDM if Inno B fired first after the warnings. The reason is because they have knowingly followed somebody despite warnings. Because of this they have no right to fire first until they are hit/shot at.
•If Inno A is being followed and gave the proper warnings and Inno B still follows, Inno A does not have to shoot/kill Inno B. That is a judgment call on Inno A’s part. If Inno B were to kill Inno A before Inno A fired, again this would be RDM
•Innos should never try killing Detectives. That certainly constitutes RDM. The only time an inno should ever kill a detective is if they start mass RDMing and even then I myself would not. The only reason why I would encourage it is because the RDMing could ruin the entire round. Again I stress this should be an absolute last resort to prevent the Detective from killing an entire server.
Here is the problem. It's going to be really hard to tell who shot who first in this instance and precisely how close the shot was. It is possible that Nexus jumped the gun but it is also possible that you shot directly at him, allowing Nexus to kill you in self defense. Its something we can't verify without actual witnesses or video evidence.
Considering that this really doesn't involve any kind of abuse of admin powers, or and the admin breaking the rules, it cannot be considered abuse.