I reviewed it as well. The incident in question takes place just after tick 113000. From your point of view, the RED Pyro was indeed moving and rebelling before a repeat was even mentioned or asked for. You were correct in killing him. But from the point of view of the rest of the game, it came across that you killed a pyro who was moving during a repeat. Your POV was the only one that saw the pyro rebel before a repeat was asked for. So yes, the slay was not correct. However: I do not see this as blatant donor abuse.
From Acer's point of view, and most of the server it seemed, you killed an innocent who was waiting for a repeat. Acer wasn't abusing his donor; he honestly thought you had killed an innocent and did what he could to fix the issue. The vote passed as well, so most of the server agreed that it seemed like you freekilled. You can also hear this in the voice chat and in the text chat about how the rest of the server had agreed. He wasn't slaying you for no reason or because he was upset, but because he thought you had broken a rule. I'll leave this to the admins to decide though.