Etymology
From Middle English merci, from Anglo-Norman merci (compare Old French merci, mercit), from Latin mercēdem, accusative of mercēs (wages, fee, price), from merx (wares, merchandise). Displaced native Middle English are, ore "mercy" (from Old English ār "mercy, grace"), Middle English mildse "mercy, clemency" (from Old English milds, milts "mercy, kindness").
I think you're comparing the Latin words. I mean in English of course. Not in Greek or Latin.
I will claim a mea culpa on the mercy / miserere. That's what I get when I use my (decrepit) memory rather than looking it up.