frustration

Hi, all

This is from <The Bat> by Jo Nesbo.

"Murderers are generally pitiful sorts, and it’s seldom difficult to point to at least ten good reasons why they turned out as they did. So, usually, what you feel most is frustration. This probably still sounds a touch sentimental . . ."

The inspector Harry is talking about his job catching murderers.
I'm not sure whose emotion is "frustration". Is it belong to murderer or we(inspectors)?

People in general, I think, and probably the police specifically. Another way to say it would be "It's frustrating that they can't . . . ."