Simply put, assassination is, by far, the most moral tool in conducting military conflicts between nations
First off, let’s dispense with sugarcoating and linguistic chicanery: the point and purpose of war is to kill people and break things. That destruction may be in the service of any number of strategic goals, but the point of going to war is to bring martial force to bear. For offensive, or defensive reasons, nations unleash martial force when there are people they wish to kill, and/or things they wish to destroy. The fundamental nature of war cannot be avoided.
Demonizing assassins makes no more sense than demonizing any other class of combatant; once combat has begun, it hardly matters if the thing that ends your life is a bullet fired by a sniper, or a bomb dropped out of a plane, or poison in your glass. That assassins are often associated with trickery, stealth and unfair fights is hardly a mark against them; if the defender sees an attack coming, can plan properly to counter it and gets a fair fight, generally the attacker has failed at one point or another in their strategy or tactics.
Assassins are, properly, specialized soldiers.
Now, that being dealt with, let’s move on to the second point. Namely, the fact that while wars are to be avoided by reasonable means, once conflict has been engaged in, the aim is to win and, in order to prosecute as moral a campaign as is humanly possible, to minimize civilian casualties. In this respect, a skilled assassin can actually do more to save lives than a hospital full of doctors. A bullet, cellphone bomb, or poison in the right place at the right time, have the potential to eliminate a target without any innocent victims being caught in the attack. A truly well conceived and well orchestrated assassination can, likewise, hamstring an opponent and enable pitched battles to be ended much more quickly and cleanly.
Now, some might object that assassination is likely to start conflict as well as prevent/contain/limit it. This is certainly true, and WW I serves as a convenient example. But just like the use of any other sort of military force, assassination is not good in-and-of-itself. Once one has decided to initiate military conflict, however, there is no more humane method of prosecuting that than by targeted assassinations.
For these reasons, if there is going to be a military conflict, whether limited or total war, assassination is the refinement of war to its most moral form.