Marcus' expression drops in disappointment as his former king completely ignores both his and Nightfall's attempts to get some sort of rational explanation for his actions. As the initial shock wears off, the reality of the situation sinks in... Rust is off his fucking trolley. As the man he had once looked to for guidance continues to display his insanity with even more of his paranoid ravings, the metalflesh simply shakes his head in resignation.
"Rust... you should leave now."