Using thermite against the enemy and enemy positions in war is legal and is neither a violation of the Geneva conventions or even of standard military conduct. It is illegal, however, when the Russians use thermite (as they have been) against civilians.

The Ukrainians have been using it exclusively against military targets, mostly for area denial. I would be doing the exact same thing in their place, as distateful as it is. I do not care about distasteful, however. As I would, the Ukrainians care about expelling the enemy from their home. Using thermite is the right thing to do to achieve that goal, and the correct way to use that tool.