[CaptainHaplo]

Legally speaking - if you know of a murder that is about to occur - and do not act in means sufficient to stop it (or at least with means sufficient to be held to have acted in good faith with a reasonable chance of success at stopping said murder) - then you are considered culpable and there are many convictions of people for what is called ACCESSORY to Murder.

If you know of one, and try to talk the person out of it - and fail - you did not do enough. If you called the cops before hand - either as soon as you learned of the plan or within time for them to stop it - your covered. If you knew yesterday but called the cops 5 minutes before the trigger was pulled and they could not get there - your culpable under the law.

The failure to act in the common good to prevent a murder is considered to be tacit approval and assistance to the murder act.

If you have no prior knowledge, then physically struggle with the murderer, are unsuccessful and then turn that person in as soon as possible, then you would be fine. If you see Joe loading his gun while drunk and talking about how he is going to go shoot his ex-wife - and you do not take action - or you take INSUFFICIENT action (within reason) - then you are morally and legally liable if he goes and kills his ex-wife.

If you cannot see where you have a moral and ethical duty to prevent a murder then there is no point in your original question - since a soldier wants to fight a war to end it, while an assassin wants to murder for money, and a merc wants to do a job to get paid.

Edit - to clarify - a soldier doesnt "want" to fight a war - but when it becomes necessary - then yes - they want to fight it - to gain its completion and return to peace. That is, at least, 98% or more of them I served with. You do have to occasional whack job - but they are easily removed from combat ops for the good of the platoon.