For music it's illegal unless the author permits it. For YouTubers making music, it takes away their revenue stream if they use ads (if they don't you're probably morally in the right), for pros that are uploaded to YouTube by someone else in the first place, I'd say it's more morally correct to download the video than to assist the person who uploaded it to YouTube in the first place, but it's still objectionable since the creators don't get any of the revenue. However, legally speaking, it's ok to download them, since in that case it's the uploader and not the downloader breaching the law. There are YouTubers who make content specifically meant to be downloaded, such as effects on green backgrounds for video editors (have used some myself)
I don't know the tool, but I know two other tools that I use for downloading videos (all legally and for fair use), and they're both trusted. Looking at it, I have no reason to believe your tool isn't either. Since the videos on YouTube and Tumblr reside on Google's and Tumblr's servers, them being infected is a risk I'd call almost less than 0. Think about it, a malicious file being uploaded to Google's servers, spreading and infecting all og Google? I'm sure they'd like to avoid that, and thus make sure no such thing happens. Of course with computers there's always a risk, but this one is minimal at most.
Yep.