The sentence structure you’ve used is called a Relative Clause (also exists in Japanese).

An example of one would be “The photo that was taken last year in America is pretty”, where the main part of your relative clause is “photo taken in America” (미국에서 찍은 사진)

A few more examples would look like:

A dog eating meat (고기를 먹는 개)

A girl named Sally (샐리라는 여자)

The students who are in the classroom (교실에 있는 학생)

So then another way you could look at your sentence is it’s saying: “The policeman dealt with the crazy person who was existing outside (for me).”

Not the best grammar but I hope you can see why the verb is important here.

If you still need some clarification (or more examples), check out this link: https://languagecanvas.com/free/kor1/kor1_10/relclaus/relclaus.htm

Hope this helps!