Killing in cold blood suggests he had another viable choice. He did not. Without the plague he could have chosen to surrender himself and the women to the "mercy" of the government. The plague meant that he couldn't even allow an arrest. As I said earlier, technically murder, but a necessary killing. It would be viewed as cold blood by the islanders because they are unaware of, and unable to accept, the existence and consequences of the plague.
What doesn't fit is that this possible situation wasn't considered before it occurred. Venturing out of isolation for any reason carries the risk of exposing someone, innocent or otherwise, and the only option, other than adding them to the expedition and forcing them to leave the island, is to kill them and burn the body. The explanation as to why Emily stayed in her old house demonstrates that all three were well aware of the risk.
Another aspect is that this ongoing discussion deals with the killing of the inquisitor as the sole responsibility of the MC. It is not. Maria and Emily decided that, rather spending the night in her room, Emily would leave so Maria and the MC could have private time, alone in the house. Had they made a different decision the inquisitor would still be alive. They are equally if not more responsible for the killing of the inquisitor, and both should accept that and share the MC's burden.
