I - Rules
(see FAQ 1.3, p. 17)
|Effect:||Play after rolling the dice in a melee attack.
The hero chooses the results of that attack roll, placing those sides of the dice faceup, except that the hero cannot change the facing result of the red die.
How does this work when the attack involves a reroll, such as an aim or dodge?
Killing Blow works on the first roll. A re-roll negates Killing Blow's effect.