As Ken mentioned, it's not public domain but because it's on Youtube, anyone can view it and, if applicable, may link to it and embed it on another site. But:
You may access Content for your information and personal use solely as intended through the provided functionality of the Service and as permitted under these Terms of Service. You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content. You shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content.
Although:
You also hereby grant each user of the Service a non-exclusive license to access your Content through the Service, and to use, reproduce, distribute, display and perform such Content as permitted through the functionality of the Service and under these Terms of Service. The above licenses granted by you in video Content you submit to the Service terminate within a commercially reasonable time after you remove or delete your videos from the Service.
Whether it's Fair Dealing (known as Fair Use in the USA) is debatable. If it is Fair Dealing then a lot of the content on one newspaper web site or broadcast on one TV news channel could be taken and reused by another newspaper or TV news channel. And how often do you see that? However, in this specific case, it may meet the requirements of Fair Dealing although some news outlets failed here.
At least two people and at least two security cameras recorded video of the shooting and several more people shot video after the fact. At least one of the people who captured the shooting has been
interviewed by
newspapers. I believe unedited video was sent to the SIU.
(Edit: added some Youtube quotes)