I'm not sure that makes a lot of sense evolution-wise, though I like your idea. Animals that don't instinctually care for their young have young that can survive independently. Animals that have defenseless young don't tend to abandon them the way OP says is the norm for her species. Sure, a few are bad parents, but that removes those instinctual behaviors from the gene pool, reinforcing instinctual good parenting behaviors. If that wasn't the case, they never would've made it long enough to become civilized.

I suspect this is something supernatural, that their ancestors were normal humans that became possessed by something in order to produce these unnatural behaviors.