Writing to Fox(or to the show creators) can't hurt either though. If that's the only thing that you CAN do in response, then do it.
You can also tweet to the producers, the stars, the show twitter, the channel twitter...
You can post on the facebook pages of all of those.
You can post on the Instagram pages of all of those.
Etc.
Use social media to share that it was unacceptable to show the beheading of a PET on the show in your opinion. Write calm, reasoned posts too, not giant all-caps shouts about SNAEKS R UR FRANDS! Point out that millions of reptiles are kept as pets, or that showing the killing of an animal kept as a pet is not good programming, whether it's a snake or a kitten.. these will possibly be read as a coherent response. After all, you're not saying they should cancel the show and charge everyone with animal cruelty... you're just wanting to point out that it WAS a pet, and unacceptable to kill pets for no reason.
Try not to get emotionally upset as much as possible, because that won't help, either you or the reptiles. But it's upsetting. It's promoting cruel behavior. It's vilifying reptiles.