The text on the card is:
"In the most desperate situation, save the life of yourself or any adjacent Doomtrooper.
Effect: Recover one health. In addition, roll [two red dice]. For every hit, one additional health is recovered. You cannot go above the starting maximum. Discard after use."
Can you use the coagulant auto-injector after your doomtrooper takes fatal damage?
Example 1: my doomtrooper has 2 health remaining. An attack causes him to take 2 damage. I immediately spend the auto-injector to recover 1 health (plus some more based on a dice roll).
Since the card mentions you cannot go above the starting health, obviously you must be allowed to use the card when you’re not at the point of death. But, it also says you can use it to “save the life of yourself”, which suggests you can use it in response to a fatal wound.
A further complication: what about “overkill”?
Example 2: my doomtrooper has 1 health remaining. An attack causes him to take 4 damage. I immediately spend the auto-injector and roll 2 hits. Is my doomtrooper still dead (1 health, plus 3 healed is 4, but he took 4 damage), or does he come back to life with 3 health (the attack kills him, his health is now 0, and then he heals 3)? Or something else?