Step-by-step explanation:
The officer is supposed to find the criminal. That is (technically) his job. A detective does more in the searching part, though.
Also, a "lamb to the slaughter" means that some prey (lamb) is going to immediately get killed/ slaughtered. Assuming that the criminal is the "lamb" and the "killer" is the officer, the irony is that the officer can not kill the criminal. That the criminal isn't exactly the prey and isn't "sent out to die" .
*I may or may not be right.*