When a reader complains something is "unrealistic" what they mean is they were suddenly reminded that they were reading a story, that none of the characters they were reading about actually existed and that none of the events they went through were actually real. Ultimately it comes down to violation of expectations: Expectations that are informed by the genre, by the work itself doing previous legwork in establishing tone and setting, and by baggage the reader carries with them when they read the story.
(It's why a Certain Type of Fantasy Reader, for example, is perfectly fine with dragons that can fly and breathe fire and wizards who can throw fireballs, but throws a fit when the setting isn't a rapists' paradise where women are treated like cattle to be bought and sold. It has nothing to do with realism, it's just how they've been trained to expect a fantasy setting to be like.)
Indeed, it's why Truth is Stranger than Fiction, as they say, because a story has to make sense to its intended audience in order to be accepted. Reality, on the other hand, is perfectly free to be inscrutable.
That doesn't mean you should necessarily disregard such criticism, though, the problem the reader is describing is a legitimate one even if they're describing it badly. But like all criticism, you as the author have to decide if that criticism is a problem with your story, or if the person making it is just not part of your intended audience.
Anyway, I'll agree that Myriam shouldn't kill Marc. Death is far too kind a fate for that little punk. /s
