I'm very sorry to hear of your situation. It is unfortunate. Like others said it may simply be a result of the specific pairing and nothing you did or could have done would have prevented it.
If you choose to cull the animals which looks like it may be the most humane option, there is an option you might consider which could allow your animals to be of some benefit. I make this suggestion not to start an argument but just as one possible option. If you know someone who is raising kingsnakes then they may be able to use them as feeders. While it seems cruel and contrary to our ways as we all care deeply about snakes, it is no more so than the act of feeding our own animals live rats. I imagine a person who likes hamsters or guinea pigs would see our practices as cruel or barbaric. However we do it because it is what are animals require for food.
In the end this will be difficult regardless. But, by doing so, you could allow another breeder who cares about their animals to provide live feeders, which is something which they may not have access to on a regular basis. Also, I imagine it is not as quick a death as decapitation, but I wouldn't think it would be terribly drawn out. If a kingsnake will take a dead snake as a prey item, then by all means dispatch them in the quickest most humane way possible. I guess I just see this as a better option than just euthanizing and discarding them.
Again, I am sorry.