No device made to kill a human is humane

Yes but some are more inhumane than others. That's why the United Nations Convention on Certain Conventional Weapons exists, which lists the following protocols:

1. Non-detectable fragments: weapons specially designed to shatter into tiny pieces, which aren’t detectable in the human body. Examples are fragmented bullets or projectiles filled with broken glass.
2. Mines, booby traps, and other devices: This includes anti-personnel mines, which are mines specially designed to target humans rather than tanks.
3. Incendiary weapons: Weapons that cause fires aren’t permitted for use on on civilian populations or in forested areas.
4. Blinding lasers: Laser weapons specifically designed to cause permanent blindness.
5. Explosive remnants of war: Parties that have used cluster bombs in combat are required to help clear any unexploded remains.

Thermite is a protocol 3 weapon. So again, while I understand that Ukraine is desperate to defend itself, using that stuff is not great.