Plot hole: The killer shows up at the scheduled appointment at 8 AM. They kill the idiot blackmailer with an overdose of morphine. Remember, that morphine that supposedly killed Thrombey in 10 minutes. Marta finds the blackmailer at 10 AM...alive, and does CPR on them, keeping them alive long enough for the ambulance to come and bring them to the hospital, even if in critical condition. So we went from "kills in 10 minutes, you can't even try to save him" to "after 2 hours, you are still hanging on"? (01:56:10)
Stupidity: Spoiler. The protagonist is a trained and competent nurse, paired with one of the greatest murder mystery writers. Neither finds strange in the slightest that after jabbing his vein with a dose of drugs 30 times the norm he is absolutely fine, not just conscious but even able to concoct on the spot a convoluted plot, speaking normally and quite at length, no trouble at all. He should be dead "in 10 minutes" sure, but it's not a time bomb. You'd think one would not be so blasé about slitting their own throat and the other would have to notice how amazingly unaffected and lucid the other appears to be minutes later. Not to mention that his plan would have never worked with the toxicology report, which should be routine in a suicide case also to assess the mental state of the person who left no note or anything behind.
Stupidity: Marta constantly screws up the investigation, but Blanc and the cops just let her keep helping in the mission. She also runs from the cops, causing damage to multiple cars. She messes up the footprints, and messes up the security tape. Why was she still on the investigation team after all of these?