Parse the word and see what was being brought forward:
Un: NOT
mercy: gracious/kind/gentle
Less: of lower quantity
merciless: without mercy
UNmerciless: not without mercy, merciful.
In other words, your invented word is just as grating and inaccurate as such gems as:
irredundant
irregardless
two words that are guaranteed to bring out the spelling/grammar libertarians in all of us.
Touche’ count it my dialect