He is a bastard. As in, might be the queen's son but he is NOT the king's! We are given nothing that might imply Victor having had more than one wife, and he and all of his other children have variations of blonde hair, meaning his wife had, by laws of genetics, had to have been blonde herself, as it is a recessive gene. Eins has black hair which is a dominant gene.
At one time, Eins may have been a good choice for the throne, but with confirmation that he was not the kings son, having a bastard as heir would become one hell of a scandal. Thus, Victor needed one of his other (legitimate) sons to be ready to become heir, asap. Thus, kicking off the plot.
