So, let's give it a shot cause I don't remember much. MC is a coward. Ran away to a mountain after discovering that his childhood friend found a lover and broke their promise. A Goddess took pity upon his cries of sorrow and *gifted* him untainted powers. MC is a monster, quite literally as his race is unknown from whence a human. Unknown to him, another him summoned, acting as a Hero to sloo the Demon. Along with this summon, a harem follows him, along with his long, long, childhood friend who promised a broken promise. Also, did I forgot to mention that this "harem" has the MC's girls in it as well? Rip.
This sounds extremely similar to "Sono Mono. Nochi ni..." the MC ran away to the mountain after the childhood friend married a hero (?) and the goddess granted him powers.