I just feed one of the pitchers half a mealworm and in 2 days it looks like this. And it smells really bad. I don't understand what I did wrong.
The stuff in the pitcher, and the pitcher itself, are rotting.
Given the color I'd say you gave it too much food. Stick with smaller flying insects, or better yet, a dilute fertilizer like Maxsea or Jack's, especially if it's going to be indoors. I use 1/4 tsp of Jack's All Purpose 20-20-20 to a gallon of distilled water, and fill the pitchers ~1/2 full (do NOT fertilize the medium). Fertilizer in the pitchers will give the plant the nutrients it requires and save you from the stink of rotting dead stuff.