Abe "Kid Twist" Reles of Murder Inc had a similar sentiment. Below is an exchange between him and assistant district attorney Burton Turkus from the book Murder Inc.

One day, I asked Reles how he ever brought himself to take human life so casually. "Did your conscience ever bother you?" I inquired. "Didn't you feel anything?"
His agate eyes showed no expression. "How did you feel when you tried your first law case?" he countered coldly.
"I was rather nervous," I admitted.
"And how about your second case?"
"It wasn't so bad, but I was still a little nervous."
"And after that?"
"Oh...after that, I was all right; I was used to it."
"You answered your own question," the Kid rasped. "It's the same with murder. I got used to it."

Looks like number three gets you over the hump.