I think the parent was trying to point out that if one side abandons principles the other side will be motivated to retaliate likewise, making the outcome of a treacherous race to the bottom becomes far more likely.
This is asymmetric warfare. Whether or not it's "good" in a vacuum is kinda irrelevant. This may enable Ukraine and its allies to make life miserable for the soldiers holding the country under siege.
What principle would this go against exactly?