Rat Killer

Each operative was named for a letter of the alphabet. As one died, each agent shifted a letter and a new agent took the name of A.
Nobody retired.
Headquarters sent out a Rat Alert. “D has turned. Kill.”
Z took the assignment. The Z always killed Rats.
A week later in Nairobi, Z pulled the trigger.
Perfect shot.
“Rat killed,” reported Z.
“That was F,” said Headquarters. “Don’t you remember the promotions last month?”
Z looked over the body. “Oops.”
Y hunted him down in Finland.
“Why?” said Z, dying.
“Not Y,” said Y. “My name is Z now.”