Mercy has really strong opinions about people cheating death or their fate. It's unknown how Mercy knows when someone’s done this, but she quickly brings it to Lorsan's attention. And then spurs him into action, one way or another. Luckily, their sense of right and wrong seems to match up pretty well, so they've made a good pair so far.
And boy does Mercy hate undead. Gets kind of ranty about it to be honest...