Have you tried deleting through the file manager? Also they are probably synced with Google Photos, so open Google Photos and try deleting from there. When you delete from Google, it also deletes from your phone, but deleting from your phone doesn't delete in Google, so it may be backward syncing. Also, make sure your trash is not full.
To delete from file manager, navigate to the DCIM folder and then choose the subfolder with the videos and delete it.
