Please help. Why is your black rat snake dead?

Answer 1

I would try not to beat yourself up too much over this, snakes can be very difficult to keep, especially if you are a beginner. Some times it can be the size of the tank you keep them in, the larger the snake the larger the tank. Also snakes can stress very easy, load noise can be distressing to any creature. It may just have had an infection, you could not possibly know that as snakes don't run fever how would you know. Being basically a very docile snake, its lethargy wouldn't have been easy to spot. You did give it live food? if the mouse was a live when you put it in you can see by what happened to the mouse what stress can do.

I have added a link in realted source material below with detailed handling on Black Rat Snake.

Good look in the future, don't let one setback put you off, snakes are fascinating pets.