The Paradox of Simultaneity: Who is the Killer?

Let's change Einstein's thought experiment on simultaneity in the following way. Let's consider a train moving from A to B (left to right) and a passenger Marcello in the middle point M of AB. A and B are the end and respectively the beginning of the train. Assume that in the train at the joints A and B there are Alex and respectively Barbara carrying each of them a gun of same caliber and bullet speed. Simultaneously, according to an observer O$_{t}$ who stays at the midpoint M in the train, Alex and Barbara fatally shoot Marcello in the heart. Therefore according to observer in the train O$_{t}$, both Alex and Barbara are guilty of first degree murder, since both their bullets penetrate Marcello's heart in the same time. Therefore Alex and Barbara are both killers. Let's consider another observer O$_{e}$ on the embankment, who sits at the midpoint M' which coincides with M. Similarly on the embankment the points A' and B' coincide respectively with A and B. According to the observer on the embankment, O$_{e}$, upon Einstein's Special Theory of Relativity because the train moves from left to right, Barbara's bullet penetrates Marcello's heart and kills him before Alex. Therefore Barbara is a killer. But Alex is not a killer, since his bullet arrives later than Barbara's, therefore Alex's bullet penetrates a dead body (not a living body). According to the observer on embankment, O$_{e}$, it's Barbara who fired the gun before Alex did. Contradiction.