There is a big leap between not resisting and using weapons that are widely agreed to be abhorrent. You're framing a false binary choice here: either deploy weapons that the majority of countries have deemed unconscionable, or lose your country in its entirety.
There is also a difference between why we fight and how we fight. Fighting in self defence does not give a state the right to conduct a war in any way it chooses.
There are numerous ongoing conflicts in which we've seen states shed hard-won agreements on how wars should be fought. It's an incredibly dangerous trend which is leading us into a new era of horror.
Where would you put the limits on Ukraine's actions? What is beyond the pale?