Advocates of Zeal explain that the war between humanity and monstrosities has two sides: perpetrator and victim. By their definition, humanity is the victim, and it's hunters' duty to stand up in defiance. Followers of Mercy have a different perspective. They tend to concentrate on individual actors rather than diametrical factions. The Merciful don't follow abstract notions of good and evil. They understand that specific circumstances demand choices and force actions. They refuse to resort to generalizations about any side in the war: "All their kind are the same." "It's for the good of mankind." "I was just following orders." The Merciful measure individuals before delivering any justice or meting out any reward.
A hunter who acts based on Mercy seeks to understand, empathize and even rationalize with the enemy. Outright hostility establishes no common ground on which terms can be established. Unquestioned violence also diminishes friend and foe into one, with no chance for identity or change.
Of course, fighting the war against the unknown isn't any easier for understanding the enemy. Sympathy makes it harder. Anything salvageable within a creature is lost when its dark side is put down. But understanding doesn't equate to tolerance of evil committed wantonly. Sympathy isn't grounds for a stay of execution for a self-proclaimed murderer. At the very least, fighting based on Mercy convinces hunters somewhat that their cause is the right one – that they're doing their individual best.
If your character's instinctive reaction upon the imbuing is to save innocent lives, to put herself in harm's way or to learn the horror's intentions, she may be a follower of Mercy.