I have spoken with both the UA and players involved, and after discussion I do not find this PK to be valid. I very much agree with your last sentence, "this feels a bit out of the spirit of the rules Prometheus follows."
Let us assume that what you said is correct, an illegal action was performed upon you, and when you called out said illegal action, you were shot for it. You called a spade a spade and died. From my understanding, you did threaten to take them to report them for this event, while extremely ill advised (especially when detained), I don't find particuarly PK worthy in this specific instance. Now, once you had gone and reported it/filed charges in court, etc. I believe a PK would be valid. However, at this stage of the RP that was occurring, I find it to be premature.
Essentially, imagine a room in which there are 3 losers: Coffee, Prime, and Donzoko are their names. Coffee shoots Prime in the head, murdering him. Donzoko exclaims, "You murdered him!" Is Coffee now able to PK Donzoko for saying what just happened? Seems like ending RP before it begins. RP is not about winning, it's about creating events/stories. This PK seems to have cut a story short before it had a conclusion worthy of death.
Accpeted.