Dogs are what their owners make them into be. I don't believe there is ever a 'no reason' answer to dogs suddenly becoming violent after years of passive behavior.
Something triggers the dog to lash out; anxiety, fear, insecurity, or pain. It's the same concept with your children as it is with your snake. Just because the dog tolerates getting crawled on, sat on, its ears pulled, tail bitten, doesn't mean it enjoys it. And I can certainly see where I dog would grumble and even lash out (ie bite) when it had enough; that's what it would do with a puppy. It's just that people are softer than dogs.
Now, don't get me wrong I'm not saying that is always the case. But I am saying that dogs aren't as unpredictable as people think they are. If you know the natural behaviors of a dog then they're even less unpredictable.
Then there are some dogs that are trained by the other, whether intentionally or unintentionally to be agressive. For some it is hardwired into their genetics and takes a firm "pack leader" (ie the owner) to be sure the dog is properly controlled at all times and knows its manners.
As for the incident with the kitten; I see two possible reasons for why that dog could have killed it. First: It was unintentional on the dog's part, he was intending to play and got too rough. You didn't say if he was acting agressive (ie growling, snarling) or not, so that was the first conclusion I came to. Second: As you said it was a stray that the dog may have percieved as a threat. Some dogs tend to become very territorial and even though we see it as a cute, harmless kitten, the dog may view it differently.
Now I wasn't there so I can't say for certain what happened with the dog. But I highly doubt the dog just decided to be bad and kill a poor kitten. Dogs are instinctive creatures. They do what comes naturally and for that matter the dog could very well have gone into predator mode and saw the kitten as prey. :shrugs: So you see there
are reasons.
I've had my blue heeler since she was almost seven weeks old, which was 6-7 years ago. I know I can trust her with my snakes, she knows they're my 'babies' and if something is my 'baby' then she makes sure it is well taken care of. The only time I've ever seen her get rough with any other animal is when a new puppy is introduced; and that is easily explained by her establishing a pecking order (which is very natural, predictable dog behavior).
As for putting the snakes near the dogs, that's for each individual to decide. To weigh their dog's behaviors, attitude, etc against the snake's own quirks. The only snake that gets to go by either of our dogs is Mali and my blue heeler is the only dog that gets to sniff Mali, neither seem effected, though after first introducing my older snake Nome (she nipped the dog's nose), my dog is much more wary and won't get close to the snake.... she sniffs from a distance.
Murrey, our young, hyper dog has no snakes priviledges due to the 'young, hyper' part. I don't want him to think we're offering down a toy. So really I think it boils down to common sense.
To the OP... next time wash your hands before handling your snake. If she seems to dislike the dog or the dog scent then thoroughly washing your hands will make her more comfortable. I know Mali doesn't like purell so I dont use it before handling her, I run upstairs and wash my hands. Another thing is maybe if you're putting your hand in front of her, the action (depending on if it was a fast or slow motion) may have startled the snake. Next time try to put your hand underneath her, rather than blocking her, do it slowly and if you need to readjust her, do so, but once again make sure it's slow so she doesn't get startled.
Sorry for the lengthy post. I happen to know more about dogs than snakes, so for once I feel like I had something useful to contribute!
