> can the right to kill coexist with the victim's right to defense?

Yes it can, and it's ludicrous to suggest otherwise. Russia believes it has the natural right to reclaim what it considers to be Russian territory. Ukraine believes it has the right to be free. So everyone should just put down their weapons and come to an agreement based on these rights?

The fallacy at the heart of your argument is that there is somehow some greater single truth, and that each side agrees that it is the greater single truth, and that everyone will just peaceably agree to follow the single greater truth because it is the single greater truth. Nothing could be less human. What are we, the Borg? We're supposed to follow some hive mind?

> something simple, like tying sanctions to murders of civilians

Not even remotely simple. Define sanctions, murders, civilians. The US bombing "drug" boats in the Caribbean, are those civilians? International law recognizes that collateral damage can legitimately happen during legitimate military operations. Is the collateral damage "murder"? How far should sanctions go? Sanction enemy banks (layer 1)? Sanction citizens of neutral countries who do business with the enemy country (layer 2)? Sanction citizens of neutral countries who do business with other citizens who do business with the enemy country (layer 3)?

They don’t both have a right to kill each other! Both “defense” and “resistance” (w/r/t the goal of self determination) have precise bounds; not all forms of warfare or violence are considered justifiable under either. Much of what Israel has done in the current conflict goes well beyond a charitable read of its right to defense, but this doesn’t imply that all defense adaptations are illegitimate.