Are non venomous snakes bad for cats?

Bad idea. The cat will be trying to kill the snake and the snake will be forced to defend itself. Besides if you have mice or rats ( the only thing a king snake would really be interested in eating) your cat would take care of them. This is a very poorly though out plan in my opinion.

The cat and the snake will attack each other.

Likely the snake won't hurt the cat, but if the snake is large, and its a new born kitten, the snake will kill and eat the baby.  Wild snakes don't make good pets and hard to provide the right food for them IF they even eat it.  

If you want a snake for a pet, set up a very large tank and buy a domesticated snake that is used to being handled.  Wild snakes will bite you and don't like to be handled.

The snake would not attack a cat; cats are too big for them to eat. However, if the cat approached the snake, it would get bitten.

Kingsnakes are too small to harm your cat. Your cat is more likely to kill the snake. Cats are known to kill rattlesnakes so you already have a good pest control agent. Snakes that you bring into you house may wind up either escaping back to the wild or they may crawl into a hole or crevice somewhere inside your house. They are not pest control agents. If you want a kingsnake as a pet, then you need to put it inside a tightly cover cage with ventilation (snakes are great escape artists), and feed it mice and give it water while keeping it warm.

More likely that your cats will attack and kill it. The cats should be sufficient pest control.

I don't think anacondas are venomous.  I think you're good to go.