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. Drosselmeyer invented the Land of the Dolls by accident
Drosselmeyer spent many years between the back story and the main story trying to find ways to reverse the Nutcracker's condition through his whimsical inventing and magic. But because he's Drosselmeyer he ended up just making more living toys and candy and decided to let his nephew be in charge of it because.....because what the heck else do you do with them?