Reputations are interesting things. Plenty of breeders have a bad reputation but it stays among other breeders and the buyers never hear the rumors. Others create a small bubble of breeders who all agree on unethical or low quality breeding practices. They then use their collective power to call everyone else ignorant or unreasonable if their practices are questioned.
There are some REALLY unethical breeders out there. Personally, I’d want to see a scanned hard copy of health testing, contact information for 2-3 buyers (recent, one that has an adult-aged, and another who has had a dog for 10+ years), contact information for their vet, and full vet records on the puppy.
A breeder willing to ship a puppy like this is probably the biggest warning sign I can think of unfortunately.
There are few things more exciting than getting a puppy and I really don’t want to seem unnecessarily negative or mean. I would just hate for you to end up with a puppy with an unstable personality or chronic health problems. Even if the breeder offers to take it back, most owners choose to handle failing health rather than return a beloved member of the family.
“Killer” sounds like an unusual exception (was he a late neuter by any chance?) but the vast majority of puppies can be accurately evaluated by the time they are ready to go home. I can remember meeting my boy’s litter at 12 weeks and their personalities then were 100% spot on.
Is the breeder asking you questions about your lifestyle, fencing, etc.? Has she called your vet or any references? A good breeder is really invested in where their puppies end up.