Can I ask where your parents are in this situation? Being that you're a minor one of them should contact the authorities and have charges brought against this immature, cruel 35 year old step-brother. He should not be allowed in your house at all.
First get charges placed against him then take a copy of the police report and have one of your parents file a case against him in small claims court for the cost of the animal. Maybe if he has to face a judge and explain his actions he might think twice about it.
I know if someone in my family had done this I wouldn't hesitate to do everything I could to get justice for the poor defenseless animal he killed. There is something seriously wrong with a person when they can commit an act such as this. I don't feel sorry for him or his spider because he intentionally placed the animal in a position to get hurt especially knowing your fear of spiders. Like someone else posted; we can't control our initial reaction to something we fear.
What he did to your snake was not done out of fear; it was done out of pure cruelty and evil. It's beyond me how anyone can bring harm to any animal.