Continuity mistake: When Eggsy is in the interview room at the police station, in a wide shot the tape recorder is on the table by itself, with nothing around it. Eggsy demands a phonecall, the police officer leaves, and Eggsy then reaches beside the recorder and picks up a cordless phone which has appeared from nowhere. (00:16:45)
Continuity mistake: In the end credits scene, when Eggsy appears in the pub, Dean is holding a pint glass in his hand, but in the next shot, the glass is suddenly on the table.
Answer: It was to test whether the candidates were willing to do whatever it takes to get the job done. If they were willing to shoot the dog they had been caring for, it means they would be willing to make a tough choice in the field to complete a mission.