Most people wouldn't suspect it, but Jim liked to read. Not anything he had to read for school, mind – if somebody told you to read it, it must be boring – but some books were interesting. Lately, Jim's favourite book is one about poisons. (Carl's death has to look like natural causes.) Belladonna would be too obvious, what with the dilated pupils. Strychnine would show up in a tox screen. Clostridium botulinum, however, would not – it's metabolized far too quickly.
When Jim hears that Carl has a swim meet in London, he decides to make his move. The team is staying at a hotel, and Jim waits until they're at dinner before sneaking into his room. He reaches into his pocket and pulls out the test tube of C. botulinum he nicked from the university. The bacteria live in a medium that resembles chicken broth, and taking care to cover his nose and mouth, Jim repeatedly dips a cotton swab in the broth and then into the eczema cream. He replaces the lid on the test tube and sneaks out the rear of the hotel.
The next day, screams echo through the pool as Carl drowns. Unnoticed by the panicked crowd, Jim darts to the locker room in search of Carl's shoes. He wants a trophy.