Batman had many trainers in his quest to become what he needed to be. At the time, David Cain was one of the worlds' premiere assassins, and thus Bruce learned from him. Though he didn't need to learn how to kill, he did want to learn how. Eventually, Bruce surpassed Cain's abilities.
That wouldn't be the last time they met though, as Cain's eventual daughter, Cassandra, would eventually become Batgirl, and earn Batman's trust completely, something her father was eventually very proud of.