Here's a perfect example of the saying "f-ck around and find out." This suspect had some sort of weapon in his hand and seemed to be toying with the cop, making movements that suggest he could turn aggressive at anytime. It did seem like for a minute he was sorta kidding with the cop, but the cop has to take every situation pretty seriously if the suspect has any sort of weapon at hand.

The cop tells him numerous times to put it down, but the guy doesn't listen and keeps making small movements. The cop warns that he will shoot the guy and the guy still does not listen. So, the cop shoots the guy.

We don't even know what sparked this incident in the first place because we don't have any context on this video, but from what we saw, the suspect f-cked around and found out.