- But why should anyone want to kill you?
- Because I'm what you would call a professional spy.

8 seconds sound clip from the The 39 Steps (1937) classic radio play.

You can hear this line at 00:08:45 in the radio play.

Quote context

[...]

- What's this all about? You said you were in trouble.

- You don't believe me, do you?

- I believe you're slightly hysterical.

- If I am, I have good reason.

- I was the one who fired those shots in that theatre tonight. I had to do something to create a disturbance. I had to get away from there!

- There were two men in that theatre - who wanted to kill me. No, you've got to believe me.

- But why should anyone want to kill you?

- Because I'm what you would call a professional spy.

- A spy? Whom do you spy for?

- Anyone who pays me.

- I suppose you're here to dig up some big state secret.

- No. I'm here to save a secret. For England.

- Oh. Go on, please.

- The agent of a certain foreign power is on the point of obtaining a secret vital to your air defence.

[...]