@Abedeus Immoral?
I see, so you would rather have a proper armed intervention which could incur heavy losses on both sides, injuries and amputations in some cases, on those that managed to survive.
Instead of weakening the enemy soldiers, reducing losses on both sides and allowing for the peaceful capture of possbily a squad or even a battalion. Of which all these soldiers could eventually be returned to their countries with some or no traumas(depending on how they are treated as prisoners).
With all due respect, please don't exarcebate this whole situation by making comparisons with the spread of lethal contaminant agents in real life. All that it takes to change an appearently unethical action into a peaceful solution is a mere change in perspective.
THe reason why it's not bad enough to be considered a War crime is because everything she did can be easily reverted and the damage caused to the enemy troops is only temporary.
This here is a case where "the goals does justify the means".
I'll ask you to read some or our actual war crimes, and compare with what our loli MC did, and then come back here and say they are one and the same. Hell even normal military operations can have far worse results than this.
Freaking Cao Cao spread a disease though many villages to weaken both enemy soldiers and damage the supply route for the allied Shu and Wu forces. Unlike here that disease was a lot deadlier, the germans and japanese spread deadly gases killing every living being in a wide area, these are actual war crimes. They caused lasting damage to a whole region and to both enemy troops, innocent civilians and animals. Kaoru's poison will affect the enemy, and will not create permanent damage either, with the request of reinforcements and more supplies these solders could improve their situation, and in the end the real damage will only be caused to their morale.