Hmmm, GREAT question! Some thoughts on OP & SW85's response:
First, OP is correct -- it PROBABLY didn't happen that way in real life. But if it had occurred, just as the movie depicted, I don't think what the officer did was so out of line as to make him guilty of murder. Some ruminations:
The Titanic was a passenger ship, not a navy warship. The officers were alone, in the middle of the night, with an increasingly unruly mob on their hands. The entire reason guns were given to the officers -- in real life, and I believe in the movie -- was to help keep order in light of the fear of order breaking down and more, not less, deaths. That was ultimately why the passengers were shot - they appeared to be rushing a lifeboat, which could lead to a swamping and more deaths.
SW85 writes, as to the shooting, to make it justified, "it must be in response to some unjust aggression on the part of the person so killed. This aggression must constitute a serious threat to the life or safety of the person enacting the justifiable homicide, or to the life or safety of a person or persons under his protection."
--the passengers as a whole were "under the protection" of the officers charged with keeping order. Rushing a lifeboat certainly jeopardized the loading of the boat, and/or endangered the "women and children first" process which the officer was employing.
- SW85 continues, "Second, the will must be in line with that intention: it is not enough for me to shoot and kill someone while defending others; I must do it with the intention of defending others. I cannot kill you in a moment of rage because you are attacking my family; I must act to defend my family."
--We can't really say what happened here, because -- after shooting the passengers -- the officer then blew his own brains out (more on that later).
- SW85 adds, "And third (related to the second), the amount of force used must be no greater than appears to be necessary, to the degree that you can control it. Obviously it would be preferable to incapacitate such a person; but if you attempt to do so and accidentally kill them in the process, you are (probably) blameless."
--I don't know if this were present here, too. On the other side of the ship, the mere threat by the second officer -- who waves the crowd back, accompanied by a threat to "shoot down like dogs!" anyone who fails to do so -- is certainly preferable (note: great scene; line delivered chillingly -- and the movie shows that the gun is unloaded, too!)...but the fact remains that the shot passengers appeared to be rushing the boat.
- SW85 writes, "If I recall the movie right, the person shot was acting aggressively because he and the other lower-class passengers were being forbidden access to lifeboats simply by virtue of their class. Insofar as this is unjust, I think it is legitimate for such a passenger to act aggressively against the officers, who are unjustly restraining them."
--Hmmm, I didn't get that at all from the movie -- I though the officer was justifiedly restraining men, as he was trying to load women in an organized fashion, and got rushed by several men who disagreed with what he was doing.
- Finally, the officer doing the shooting (First Officer Murdoch) was the same officer who'd been on the bridge, "on watch," when the ship hit the iceberg, and he was the officer most directly responsible for driving the ship into the iceberg (we can debate the culpability of the captain, the owner, etc., but the fact remains Murdoch was on watch when the collision occurred). His emotional state -- which surely included extreme guilt -- likely lessened his culpability for shooting the passengers in my opinion. In short, he pobably just wasn't in his right mind. After shooting the passengers in the film, he then blows his own brains out -- not the actions of someone who is thinking rationally.
That film - and the real events depicted therein - offers VERY fertile ground for moral conundrums. For example: The managing partner Bruce Ismay saves his own life, after signing off on the lack of lifeboats. Was that justifyable? Was he morally obligated to give up his lifeboat seat to someone else? If so....was anyone else so obligated? Anyway, those are other threads, sorry.