ok am gonna assume you are sincere in wanting to know the difference. 

1.    Imperative: In this context, an imperative means something is necessary or unavoidable due to circumstances (e.g., self-defense, protection of innocent lives). This doesn't necessarily imply moral approval or justification for war.

2.    Justification: Justifying war implies that the act of waging war is morally acceptable, which can be problematic. War often leads to significant human suffering, and it's essential to question whether the ends (whatever they might be) justify the means.

If I am forced to kill then I will. That is different than saying it was right.