[HiTech RedNeck]

I was going to do a humble mea culpa, but I double checked on Wiktionary. Which says

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.

[markomalley]

I use the Greek because we should concern ourselves with the languages used by the apostles and evangelists when they wrote the Scriptures.

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.