question:  Why did Frank shoot his new client at the beginning of the story?
context: Frank Pembroke sat behind the desk of his shabby little office over Lemark's Liquors in downtown Los Angeles and waited for his first customer. Therefore, it was with a mingled sense of excitement and satisfaction that he greeted the tall, dark, smooth-faced figure that came up the stairs and into the office shortly before noon. Opening the desk drawer, which was almost empty, Pembroke removed an automatic pistol fitted with a silencer. Pointing it at the amazed customer, he fired four .22 caliber longs into the narrow chest. The series of events leading up to Pembroke's present occupation had commenced on a dismal, overcast evening in the South Pacific a year earlier. Standing on the forward deck, Pembroke was one of the first to perceive the peculiar odor and to spot the immense gray hulk wallowing in the murky distance. Then the explosion had come, from far below the waterline, and the decks were awash with frantic crewmen, officers, and the handful of passengers. Pembroke came as close to being a professional adventurer as one can in these days of regimented travel, organized peril, and political restriction. Lacking such loyalties, Pembroke adapted quickly to the situation in which he found himself when he regained consciousness. The wallet in his pocket contained exactly what it should, approximately three hundred dollars. One in particular, an attractive woman of about thirty, tossed back her long, chestnut locks and gazed up intently at Pembroke as he passed. "You're not casual enough, for one thing," said Pembroke, deciding to play along with her for the moment. "Incidentally, just where are we, anyway? But as he left the shop he began checking off every west coast state, city, town, and inlet. "Which way is it to San Francisco?" asked Pembroke. Pembroke watched him hurry off to service a car with a sense of having been given the runaround. "All cargo ships, I'll bet. Going to the bar, he recognized one of the Elena Mia's paying passengers.
output:  He wanted to collect the body as evidence of an impending attack.
options:  He wanted to collect the body as evidence of an impending attack.  The man who walked into his office was dangerous and Frank needed to protect himself.  He wanted to hurt the people who caused the Elena Mia to sink.  He had put out an ad for people who wanted to get shot to escape life as it is.