Maybe not an answer...
In the FAQ, in section/subsection: 4. Assault - 4.1.1. Ranged:
Defender units killed in melee go the hospital, do all other "killing methods" (fire master,
blood stones, etc.) removes them from the game immediately?
also from Invader's rulebook
Assault, pg16 wrote:
All Invading Units killed during any Stage are returned to the
supply (not the bag). All Defending Units killed during the Ranged
Attack are also returned to the supply. All Defending Units killed
during the Melee Combat are moved to the Hospital temporarily.