I always saw the key theme of the malifaux world centering around Malifaux as this big meat grinder, where human life is just cast away in droves to repower the soulstone veins. The Neverborn seem like the resident natives that occasionally let humans in and grow in power to kill tons more people and enrich the soulstones.
My impression was that December was a cannibalistic human who got so powerful, he became troublesome for the Neverborn, and would have seized that power for himself. So the Neverborn had to put him down. If December was a Neverborn, I don't think the other Neverborns would have cared. They only seem to get mad when humans are uppity.
If that's not what happened, then I'm gonna have to go with my Aqua Teen Hunger Force theory:
"THOUSANDS OF YEARS AGO, before the dawn of man as we knew him, there was Sir Santa of Claus, an ape-like creature making crude and pointless toys out of dinobones and his own waste, hurling them at chimp-like creatures with crinkled hands regardless of how they behaved the previous year."