Although Instagram gives an option to “Save the video”, but it doesn’t allow you to download videos you find on your feed or on other users’ videos. So, it’s illegal to download Instagram videos in order to alter or share them as your own. You can only download videos for personal use. If you violate copyrights, no one will be responsible for any actions taken against you.
Moreover, it completely depends upon what will you do after downloading Instagram video. It is legal to download Instagram Videos if it is not used for commercial purpose without permission from the original creator. As it comes under “Copyright Disclaimer Section 107” of the copyright 1967, allowance is made for fair use for purpose such as criticism, comment, news reporting, and scholarship.