You are expected to believe that, because that's what Baine, who you're more fond of than most, says, as @Mehrunes has again reminded. The Warchief had supreme authority over the Horde in all regards. There has never in the game been a Horde judicial system. There are no Horde judges, there are no Horde courts. The Horde is, or rather was, an army with a state, not a state with an army. Thrall didn't try Burx, Garrosh didn't try Krom'gar. They killed them on the spot and were fully in their right to do so. Past that, any state in the world would deliver either life imprisonment or the death penalty for equivalent acts by those bound to it.
I've seen the cinematic. The Derek model is the Daelin model used in the earlier Jaina cinematic with slightly different textures and a different head. The knife is a part of both models. The reuse and focus on it is symbolic - hence why we center on it as Derek goes on about how he was going to be used to kill them. It's not confirmed that that's the method. Jaina herself ponders if he's a suicide bomber. It's no more a suggestion that that's how he'd go killing (since he'd freed before he's ready anyway) than it is a suggestion that he was still controlled, which you'll recall I told people back when it came out was entirely off-base and he was in control of himself.And you should know by now camera focus is not "incidental" in cutscenes by Blizzard, they put immense attention to detail in each shot to cram as much storytelling as possible into a short time frame.