The psychiatrists say it's because he believes people are plotting against him, and so he strikes back and tries to kill them.
9 seconds sound clip from the Beat the Devil (1953) movie soundboard.
You can hear this line at 01:10:54 in the Blu-ray version of the movie.
Quote context
[...]
- It's no use, Billy, my trying to protect Harry any further.
- I may as well tell the whole truth.
- Captain, it grieves me to confess this, but in point of fact, my husband has an illness of the mind.
- The medical word for it is paranoia.
- On occasion, he displays homicidal tendencies.
- For heaven's sake, woman! What's the meaning of this treachery?
- Believe it or not, Harry, I'm doing it for your own good.
- He knows! He saved my life. He'll tell the truth.
- I wouldn't contradict the lady.
- You wrecked my ship! You tried to kill the passengers!
[...]