Objection: No one has ever died "because others were too slow to react" to a potential threat. The only one(s) responsible for violence are those committing the violence. Putting that sin on someone else in invalid. Willful commission of murder (or assault or rape or anything else) is the sole responsibility of the person(s) committing the act. In the case of unwillful commission, such as someone forcibly someone else into committing a heinous act, the willful person(s) is the one responsible. In no cases, in absolutely zero cases, is anyone responsible for not preventing a heinous act. That decision is left to the "uninvolved" people themselves. He or she or they may decide that the lack of action was incorrect, or they may decide otherwise. But the judgement is left to them.

So, what about soldiers? Well, in some cases, I suppose that some are forced to murder - by physical force, mental force, threats to others dear to them. But the one that picks up the rifle and kills someone remains the SOLE person responsible.

"'Cause I won't know the man that kills me
And I don't know these men I kill
And we all wind up on the same side
'Cause ain't none of us doing God's will"

Now, what about "defensive" force? Whatever. Anyone who commits the act is still the responsible party. Whether one accepts that responsibility as "good" or "evil" is immaterial. The responsibility is still on the commitant. In no case are those who "could have prevented this" ever responsible for any heinous act.