The seven-headed mouse king was created through necromancy.Mouserinks attempted to resurrect her seven sons with magic, but they Came Back Wrong with their heads attached to one body.
The Nucracker's origin story as we are told it is not quite true.The Nutcracker really, really hates Drosselmeyer - indeed, he gets angrier at mention of his uncle than he does when talking about his mortal foe, the Mouse King. But there's nothing in Drosselmeyer's story to justify this. However, we must recall that's all we know - Drosselmeyer's story. Perhaps it was he who tripped the boy, or perhaps Mouserinks was willing to negotiate, and he refused, or any number of other events.